draft-ietf-bgp-bgp4-03.txt   rfc1654.txt 
Network Working Group Y. Rekhter Network Working Group Y. Rekhter
Request for Comments: DRAFT T.J. Watson Research Center, IBM Corp. Request for Comments: 1654 T.J. Watson Research Center, IBM Corp.
T.Li Category: Standards Track T. Li
cisco Systems cisco Systems
Editors Editors
August 1992 July 1994
A Border Gateway Protocol 4 (BGP-4) A Border Gateway Protocol 4 (BGP-4)
Status of this Memo Status of this Memo
This document, together with its companion document, "Application of This document specifies an Internet standards track protocol for the
the Border Gateway Protocol in the Internet", define an inter- Internet community, and requests discussion and suggestions for
autonomous system routing protocol for the Internet. This RFC improvements. Please refer to the current edition of the "Internet
specifies an IAB standards track protocol for the Internet community, Official Protocol Standards" (STD 1) for the standardization state
and requests discussion and suggestions for improvements. Please and status of this protocol. Distribution of this memo is unlimited.
refer to the current edition of the "IAB Official Protocol Standards"
for the standardization state and status of this protocol.
Distribution of this document is unlimited.
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a "working
draft" or "work in progress".
1. Acknowledgements 1. Acknowledgements
This document was originally published as RFC 1267 in October 1991, This document was originally published as RFC 1267 in October 1991,
jointly authored by Kirk Lougheed (cisco Systems) and Yakov Rekhter jointly authored by Kirk Lougheed (cisco Systems) and Yakov Rekhter
(IBM). (IBM).
We would like to express our thanks to Guy Almes (Rice University), We would like to express our thanks to Guy Almes (Rice University),
Len Bosack (cisco Systems), and Jeffrey C. Honig (Cornell University) Len Bosack (cisco Systems), and Jeffrey C. Honig (Cornell University)
for their contributions to the earlier version of this document. for their contributions to the earlier version of this document.
We like to explicitly thank Bob Braden (ISI) for the review of the We like to explicitly thank Bob Braden (ISI) for the review of the
earlier version of this document as well as his constructive and earlier version of this document as well as his constructive and
valuable comments. valuable comments.
RFC DRAFT August 1992
We would also like to thank Bob Hinden, Director for Routing of the We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he Internet Engineering Steering Group, and the team of reviewers he
assembled to review earlier versions of this document. This team, assembled to review the previous version (BGP-2) of this document.
consisting of Deborah Estrin, Milo Medin, John Moy, Radia Perlman, This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia
Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted with a Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
strong combination of toughness, professionalism, and courtesy. with a strong combination of toughness, professionalism, and
courtesy.
This updated version of the document is the product of the IETF BGP This updated version of the document is the product of the IETF BGP
Working Group with Yakov Rekhter and Tony Li as editors. Certain Working Group with Yakov Rekhter and Tony Li as editors. Certain
sections of the document borrowed heavily from IDRP [7], which is the sections of the document borrowed heavily from IDRP [7], which is the
OSI counterpart of BGP. For this credit should be given to the ANSI OSI counterpart of BGP. For this credit should be given to the ANSI
X3S3.3 group chaired by Lyman Chapin (BBN) and to Charles Kunzinger X3S3.3 group chaired by Lyman Chapin (BBN) and to Charles Kunzinger
(IBM Corp.) who is the IDRP editor within that group. We would also (IBM Corp.) who was the IDRP editor within that group. We would also
like to thank Mike Craren (Proteon, Inc.), Dimitry Haskin (BBN) and like to thank Mike Craren (Proteon, Inc.), Dimitry Haskin
Dennis Ferguson (University of Toronto) for their insightful (Wellfleet), John Krawczyk (Wellfleet), and Paul Traina (cisco) for
comments. their insightful comments.
We would like to specially acknowledge numerous contributions by
Dennis Ferguson (ANS).
2. Introduction 2. Introduction
The Border Gateway Protocol (BGP) is an inter-Autonomous System The Border Gateway Protocol (BGP) is an inter-Autonomous System
routing protocol. It is built on experience gained with EGP as routing protocol. It is built on experience gained with EGP as
defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as
described in RFC 1092 [2] and RFC 1093 [3]. described in RFC 1092 [2] and RFC 1093 [3].
The primary function of a BGP speaking system is to exchange network The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network reachability information with other BGP systems. This network
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Autonomous Systems (ASs) that reachability information traverses. Autonomous Systems (ASs) that reachability information traverses.
This information is sufficient to construct a graph of AS This information is sufficient to construct a graph of AS
connectivity from which routing loops may be pruned and some policy connectivity from which routing loops may be pruned and some policy
decisions at the AS level may be enforced. decisions at the AS level may be enforced.
BGP-4 provides a new set of mechanisms for supporting classless BGP-4 provides a new set of mechanisms for supporting classless
interdomain routing. These mechanisms include support for interdomain routing. These mechanisms include support for
advertising an IP prefix and eliminates the concept of network advertising an IP prefix and eliminates the concept of network
"class" within BGP. BGP-4 also introduces mechanisms which allow "class" within BGP. BGP-4 also introduces mechanisms which allow
aggregation of routes, including aggregation of AS paths. These aggregation of routes, including aggregation of AS paths. These
changes provide support for the proposed supernetting scheme [8]. changes provide support for the proposed supernetting scheme [8, 9].
To characterize the set of policy decisions that can be enforced To characterize the set of policy decisions that can be enforced
using BGP, one must focus on the rule that a BGP speaker advertise using BGP, one must focus on the rule that a BGP speaker advertise to
to its peer in neighbor ASs only those routes that it itself uses. its peers (other BGP speakers which it communicates with) in
This rule reflects the "hop-by-hop" routing paradigm generally used neighboring ASs only those routes that it itself uses. This rule
throughout the current Internet. Note that some policies cannot be reflects the "hop-by-hop" routing paradigm generally used throughout
supported by the "hop-by-hop" routing paradigm and thus require the current Internet. Note that some policies cannot be supported by
techniques such as source routing to enforce. For example, BGP does the "hop-by-hop" routing paradigm and thus require techniques such as
not enable one AS to send traffic to a neighboring AS intending that source routing to enforce. For example, BGP does not enable one AS
to send traffic to a neighboring AS intending that the traffic take a
RFC DRAFT August 1992 different route from that taken by traffic originating in the
neighboring AS. On the other hand, BGP can support any policy
the traffic take a different route from that taken by traffic conforming to the "hop-by-hop" routing paradigm. Since the current
originating in the neighboring AS. On the other hand, BGP can Internet uses only the "hop-by-hop" routing paradigm and since BGP
support any policy conforming to the "hop-by-hop" routing paradigm. can support any policy that conforms to that paradigm, BGP is highly
Since the current Internet uses only the "hop-by-hop" routing applicable as an inter-AS routing protocol for the current Internet.
paradigm and since BGP can support any policy that conforms to that
paradigm, BGP is highly applicable as an inter-AS routing protocol
for the current Internet.
A more complete discussion of what policies can and cannot be A more complete discussion of what policies can and cannot be
enforced with BGP is outside the scope of this document (but refer to enforced with BGP is outside the scope of this document (but refer to
the companion document discussing BGP usage [5]). the companion document discussing BGP usage [5]).
BGP runs over a reliable transport protocol. This eliminates the BGP runs over a reliable transport protocol. This eliminates the
need to implement explicit update fragmentation, retransmission, need to implement explicit update fragmentation, retransmission,
acknowledgement, and sequencing. Any authentication scheme used by acknowledgement, and sequencing. Any authentication scheme used by
the transport protocol may be used in addition to BGP's own the transport protocol may be used in addition to BGP's own
authentication mechanisms. The error notification mechanism used in authentication mechanisms. The error notification mechanism used in
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here stresses the fact that, even when multiple IGPs and metrics are here stresses the fact that, even when multiple IGPs and metrics are
used, the administration of an AS appears to other ASs to have a used, the administration of an AS appears to other ASs to have a
single coherent interior routing plan and presents a consistent single coherent interior routing plan and presents a consistent
picture of what networks are reachable through it. picture of what networks are reachable through it.
The planned use of BGP in the Internet environment, including such The planned use of BGP in the Internet environment, including such
issues as topology, the interaction between BGP and IGPs, and the issues as topology, the interaction between BGP and IGPs, and the
enforcement of routing policy rules is presented in a companion enforcement of routing policy rules is presented in a companion
document [5]. This document is the first of a series of documents document [5]. This document is the first of a series of documents
planned to explore various aspects of BGP application. Please send planned to explore various aspects of BGP application. Please send
comments to the BGP mailing list (iwg@rice.edu). comments to the BGP mailing list (iwg@ans.net).
RFC DRAFT August 1992
3. Summary of Operation 3. Summary of Operation
Two systems form a transport protocol connection between one another. Two systems form a transport protocol connection between one another.
They exchange messages to open and confirm the connection parameters. They exchange messages to open and confirm the connection parameters.
The initial data flow is the entire BGP routing table. Incremental The initial data flow is the entire BGP routing table. Incremental
updates are sent as the routing tables change. BGP does not require updates are sent as the routing tables change. BGP does not require
periodic refresh of the entire BGP routing table. Therefore, a BGP periodic refresh of the entire BGP routing table. Therefore, a BGP
speaker must retain the current version of the entire BGP routing speaker must retain the current version of the entire BGP routing
tables of all of its peers for the duration of the connection. tables of all of its peers for the duration of the connection.
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arrive at an agreement as to which border routers will serve as arrive at an agreement as to which border routers will serve as
exit/entry points for particular networks outside the AS. This exit/entry points for particular networks outside the AS. This
information is communicated to the AS's internal routers, possibly information is communicated to the AS's internal routers, possibly
via the interior routing protocol. Care must be taken to ensure that via the interior routing protocol. Care must be taken to ensure that
the interior routers have all been updated with transit information the interior routers have all been updated with transit information
before the BGP speakers announce to other ASs that transit service is before the BGP speakers announce to other ASs that transit service is
being provided. being provided.
Connections between BGP speakers of different ASs are referred to as Connections between BGP speakers of different ASs are referred to as
"external" links. BGP connections between BGP speakers within the "external" links. BGP connections between BGP speakers within the
same AS are referred to as "internal" links. same AS are referred to as "internal" links. Similarly, a peer in a
different AS is referred to as an external peer, while a peer in the
same AS may be described as an internal peer.
3.1 Routes: Advertisement and Storage 3.1 Routes: Advertisement and Storage
For purposes of this protocol a route is defined as a unit of For purposes of this protocol a route is defined as a unit of
information that pairs a destination with the attributes of a path to information that pairs a destination with the attributes of a path to
that destination: that destination:
- Routes are advertised between a pair of BGP speakers in UPDATE - Routes are advertised between a pair of BGP speakers in UPDATE
RFC DRAFT August 1992
messages: the destination is the systems whose IP addresses are messages: the destination is the systems whose IP addresses are
reported in the Network Layer Reachability Information (NLRI) reported in the Network Layer Reachability Information (NLRI)
field, and the the path is the information reported in the path field, and the the path is the information reported in the path
attributes fields of the same UPDATE message. attributes fields of the same UPDATE message.
- Routes are stored in the Routing Information Bases (RIBs): - Routes are stored in the Routing Information Bases (RIBs):
namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes
that will be advertised to other BGP speakers must be present in that will be advertised to other BGP speakers must be present in
the Adj-RIB-Out; routes that will be used by the local BGP speaker the Adj-RIB-Out; routes that will be used by the local BGP speaker
must be present in the Loc-RIB, and the next hop for each of these must be present in the Loc-RIB, and the next hop for each of these
routes must be present in the local BGP speaker's forwarding routes must be present in the local BGP speaker's forwarding
information base; and routes that are received from other BGP information base; and routes that are received from other BGP
speakers are present in the Adj-RIBs-In. speakers are present in the Adj-RIBs-In.
If a BGP speaker chooses to advertise the route, it may add to or If a BGP speaker chooses to advertise the route, it may add to or
modify the path attributes of the route before advertising it to modify the path attributes of the route before advertising it to a
adjacent BGP speaker. peer.
BGP provides mechanisms by which a BGP speaker can inform its BGP provides mechanisms by which a BGP speaker can inform its peer
neighbor that a previously advertised route is no longer available that a previously advertised route is no longer available for use.
for use. There are three methods by which a given BGP speaker can There are three methods by which a given BGP speaker can indicate
indicate that a route has been withdrawn from service: that a route has been withdrawn from service:
a) the IP prefix that expresses destinations for a previously a) the IP prefix that expresses destinations for a previously
advertised route can be advertised in the WITHDRAWN ROUTES field advertised route can be advertised in the WITHDRAWN ROUTES field
in the UPDATE message, thus marking the associated route as being in the UPDATE message, thus marking the associated route as being
no longer available for use no longer available for use
b) a replacement route with the same Network Layer Reachability b) a replacement route with the same Network Layer Reachability
Information can be advertised, or Information can be advertised, or
c) the BGP speaker - BGP speaker connection can be closed, which c) the BGP speaker - BGP speaker connection can be closed, which
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3.2 Routing Information Bases 3.2 Routing Information Bases
The Routing Information Base (RIB) within a BGP speaker consists of The Routing Information Base (RIB) within a BGP speaker consists of
three distinct parts: three distinct parts:
a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has
been learned from inbound UPDATE messages. Their contents been learned from inbound UPDATE messages. Their contents
represent routes that are available as an input to the Decision represent routes that are available as an input to the Decision
Process. Process.
RFC DRAFT August 1992
b) Loc-RIB: The Loc-RIB contains the local routing information b) Loc-RIB: The Loc-RIB contains the local routing information
that the BGP speaker has selected by applying its local policies that the BGP speaker has selected by applying its local policies
to the routing information contained in its Adj-RIBs-In. to the routing information contained in its Adj-RIBs-In.
c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the
local BGP speaker has selected for advertisement to its neighbors. local BGP speaker has selected for advertisement to its peers. The
The routing information stored in the Adj-RIBs-Out will be carried routing information stored in the Adj-RIBs-Out will be carried in
in the local BGP speaker's UPDATE messages and advertised to its the local BGP speaker's UPDATE messages and advertised to its
neighbor BGP speakers. peers.
In summary, the Adj-RIBs-In contain unprocessed routing information In summary, the Adj-RIBs-In contain unprocessed routing information
that has been advertised to the local BGP speaker by its neighbors; that has been advertised to the local BGP speaker by its peers; the
the Loc-RIB contains the routes that have been selected by the local Loc-RIB contains the routes that have been selected by the local BGP
BGP speaker's Decision Process; and the Adj-RIBs-Out organize the speaker's Decision Process; and the Adj-RIBs-Out organize the routes
routes for advertisement to specific neighbor BGP speakers by means for advertisement to specific peers by means of the local speaker's
of the local speaker's UPDATE messages. UPDATE messages.
Although the conceptual model distinguishes between Adj-RIBs-In, Although the conceptual model distinguishes between Adj-RIBs-In,
Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an
implementation must maintain three separate copies of the routing implementation must maintain three separate copies of the routing
information. The choice of implementation (for example, 3 copies of information. The choice of implementation (for example, 3 copies of
the information vs 1 copy with pointers) is not constrained by the the information vs 1 copy with pointers) is not constrained by the
protocol. protocol.
4. Message Formats 4. Message Formats
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message size is 4096 octets. All implementations are required to message size is 4096 octets. All implementations are required to
support this maximum message size. The smallest message that may be support this maximum message size. The smallest message that may be
sent consists of a BGP header without a data portion, or 19 octets. sent consists of a BGP header without a data portion, or 19 octets.
4.1 Message Header Format 4.1 Message Header Format
Each message has a fixed-size header. There may or may not be a data Each message has a fixed-size header. There may or may not be a data
portion following the header, depending on the message type. The portion following the header, depending on the message type. The
layout of these fields is shown below: layout of these fields is shown below:
RFC DRAFT August 1992
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
+ + + +
| | | |
+ + + +
| Marker | | Marker |
+ + + +
| | | |
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rest of the message. rest of the message.
Type: Type:
This 1-octet unsigned integer indicates the type code of the This 1-octet unsigned integer indicates the type code of the
message. The following type codes are defined: message. The following type codes are defined:
1 - OPEN 1 - OPEN
2 - UPDATE 2 - UPDATE
3 - NOTIFICATION 3 - NOTIFICATION
RFC DRAFT August 1992
4 - KEEPALIVE 4 - KEEPALIVE
4.2 OPEN Message Format 4.2 OPEN Message Format
After a transport protocol connection is established, the first After a transport protocol connection is established, the first
message sent by each side is an OPEN message. If the OPEN message is message sent by each side is an OPEN message. If the OPEN message is
acceptable, a KEEPALIVE message confirming the OPEN is sent back. acceptable, a KEEPALIVE message confirming the OPEN is sent back.
Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
messages may be exchanged. messages may be exchanged.
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Version: Version:
This 1-octet unsigned integer indicates the protocol version This 1-octet unsigned integer indicates the protocol version
number of the message. The current BGP version number is 4. number of the message. The current BGP version number is 4.
My Autonomous System: My Autonomous System:
This 2-octet unsigned integer indicates the Autonomous System This 2-octet unsigned integer indicates the Autonomous System
number of the sender. number of the sender.
RFC DRAFT August 1992
Hold Time: Hold Time:
This 2-octet unsigned integer indicates the maximum number of This 2-octet unsigned integer indicates the number of seconds
that the sender proposes for the value of the Hold Timer. Upon
receipt of an OPEN message, a BGP speaker MUST calculate the
value of the Hold Timer by using the smaller of its configured
Hold Time and the Hold Time received in the OPEN message. The
Hold Time MUST be either zero or at least three seconds. An
implementation may reject connections on the basis of the Hold
Time. The calculated value indicates the maximum number of
seconds that may elapse between the receipt of successive seconds that may elapse between the receipt of successive
KEEPALIVE and/or UPDATE and/or NOTIFICATION messages by the sender, KEEPALIVE, and/or UPDATE messages by the sender.
before the sender will declare the receiver as down.
BGP Identifier: BGP Identifier:
This 4-octet unsigned integer indicates the BGP Identifier of This 4-octet unsigned integer indicates the BGP Identifier of
the sender. A given BGP speaker sets the value of its BGP the sender. A given BGP speaker sets the value of its BGP
Identifier to an IP address assigned to that BGP speaker. Identifier to an IP address assigned to that BGP speaker. The
The value of the BGP Identifier is determined on startup value of the BGP Identifier is determined on startup and is the
and is the same for every local interface and every BGP peer. same for every local interface and every BGP peer.
Authentication Code: Authentication Code:
This 1-octet unsigned integer indicates the authentication This 1-octet unsigned integer indicates the authentication
mechanism being used. Whenever an authentication mechanism is mechanism being used. Whenever an authentication mechanism is
specified for use within BGP, three things must be included in the specified for use within BGP, three things must be included in
specification: the specification:
- the value of the Authentication Code which indicates use of - the value of the Authentication Code which indicates use of
the mechanism, the mechanism,
- the form and meaning of the Authentication Data, and - the form and meaning of the Authentication Data, and
- the algorithm for computing values of Marker fields. - the algorithm for computing values of Marker fields. Only
Only one authentication mechanism is specified as part of this one authentication mechanism is specified as part of this
memo: memo:
- its Authentication Code is zero, - its Authentication Code is zero,
- its Authentication Data must be empty (of zero length), and - its Authentication Data must be empty (of zero length), and
- the Marker fields of all messages must be all ones. - the Marker fields of all messages must be all ones. The
The semantics of non-zero Authentication Codes lies outside the semantics of non-zero Authentication Codes lies outside the
scope of this memo. scope of this memo.
Note that a separate authentication mechanism may be used in Note that a separate authentication mechanism may be used in
establishing the transport level connection. establishing the transport level connection.
Authentication Data: Authentication Data:
The form and meaning of this field is a variable-length field The form and meaning of this field is a variable-length field
depend on the Authentication Code. If the value of Authentication depend on the Authentication Code. If the value of
Code field is zero, the Authentication Data field must have zero Authentication Code field is zero, the Authentication Data
length. The semantics of the non-zero length Authentication Data field must have zero length. The semantics of the non-zero
field is outside the scope of this memo. length Authentication Data field is outside the scope of this
memo.
Note that the length of the Authentication Data field can be Note that the length of the Authentication Data field can be
determined from the message Length field by the formula: determined from the message Length field by the formula:
Message Length = 29 + Authentication Data Length Message Length = 29 + Authentication Data Length
RFC DRAFT August 1992
The minimum length of the OPEN message is 29 octets (including The minimum length of the OPEN message is 29 octets (including
message header). message header).
4.3 UPDATE Message Format 4.3 UPDATE Message Format
UPDATE messages are used to transfer routing information between BGP UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE packet can be used to construct peers. The information in the UPDATE packet can be used to construct
a graph describing the relationships of the various Autonomous a graph describing the relationships of the various Autonomous
Systems. By applying rules to be discussed, routing information Systems. By applying rules to be discussed, routing information
loops and some other anomalies may be detected and removed from loops and some other anomalies may be detected and removed from
inter-AS routing. inter-AS routing.
An UPDATE message is used advertise a single feasible route to a An UPDATE message is used to advertise a single feasible route to a
neighboring BGP speaker, or to withdraw multiple unfeasible routes peer, or to withdraw multiple unfeasible routes from service (see
from service (see 3.1). An UPDATE message may simultaneously advertise 3.1). An UPDATE message may simultaneously advertise a feasible route
a feasible route and withdraw multiple unfeasible routes from service. and withdraw multiple unfeasible routes from service. The UPDATE
The UPDATE message always includes the fixed-size BGP header, message always includes the fixed-size BGP header, and can optionally
and can optionally include the other fields as shown below: include the other fields as shown below:
+-----------------------------------------------------+ +-----------------------------------------------------+
| Unfeasible Routes Length (2 octets) | | Unfeasible Routes Length (2 octets) |
+-----------------------------------------------------+ +-----------------------------------------------------+
| Withdrawn Routes (variable) | | Withdrawn Routes (variable) |
+-----------------------------------------------------+ +-----------------------------------------------------+
| Total Path Attribute Length (2 octets) | | Total Path Attribute Length (2 octets) |
+-----------------------------------------------------+ +-----------------------------------------------------+
| Path Attributes (variable) | | Path Attributes (variable) |
+-----------------------------------------------------+ +-----------------------------------------------------+
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the Withdrawn Routes field in octets. Its value must allow the the Withdrawn Routes field in octets. Its value must allow the
length of the Network Layer Reachability Information field to length of the Network Layer Reachability Information field to
be determined as specified below. be determined as specified below.
A value of 0 indicates that no routes are being withdrawn from A value of 0 indicates that no routes are being withdrawn from
service, and that the WITHDRAWN ROUTES field is not present in service, and that the WITHDRAWN ROUTES field is not present in
this UPDATE message. this UPDATE message.
Withdrawn Routes: Withdrawn Routes:
RFC DRAFT August 1992
This is a variable length field that contains a list of IP This is a variable length field that contains a list of IP
address prefixes for the routes that are being withdrawn from address prefixes for the routes that are being withdrawn from
service. Each IP address prefix is encoded as a 2-tuple of the service. Each IP address prefix is encoded as a 2-tuple of the
form <length, prefix>, whose fields are described below: form <length, prefix>, whose fields are described below:
+---------------------------+ +---------------------------+
| Length (1 octet) | | Length (1 octet) |
+---------------------------+ +---------------------------+
| Prefix (variable) | | Prefix (variable) |
+---------------------------+ +---------------------------+
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Path Attributes: Path Attributes:
A variable length sequence of path attributes is present in A variable length sequence of path attributes is present in
every UPDATE. Each path attribute is a triple <attribute type, every UPDATE. Each path attribute is a triple <attribute type,
attribute length, attribute value> of variable length. attribute length, attribute value> of variable length.
Attribute Type is a two-octet field that consists of the Attribute Type is a two-octet field that consists of the
Attribute Flags octet followed by the Attribute Type Code Attribute Flags octet followed by the Attribute Type Code
octet. octet.
RFC DRAFT August 1992
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code| | Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The high-order bit (bit 0) of the Attribute Flags octet is the The high-order bit (bit 0) of the Attribute Flags octet is the
Optional bit. It defines whether the attribute is optional (if Optional bit. It defines whether the attribute is optional (if
set to 1) or well-known (if set to 0). set to 1) or well-known (if set to 0).
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If the Extended Length bit of the Attribute Flags octet is set If the Extended Length bit of the Attribute Flags octet is set
to 0, the third octet of the Path Attribute contains the length to 0, the third octet of the Path Attribute contains the length
of the attribute data in octets. of the attribute data in octets.
If the Extended Length bit of the Attribute Flags octet is set If the Extended Length bit of the Attribute Flags octet is set
to 1, then the third and the fourth octets of the path to 1, then the third and the fourth octets of the path
attribute contain the length of the attribute data in octets. attribute contain the length of the attribute data in octets.
The remaining octets of the Path Attribute represent the The remaining octets of the Path Attribute represent the
attribute value and are interpreted according to the Attribute attribute value and are interpreted according to the Attribute
Flags and the Attribute Type Code. The supported Attribute Type
RFC DRAFT August 1992 Codes, their attribute values and uses are the following:
Flags and the Attribute Type Code. The supported attribute
values and their uses are the following:
a) ORIGIN (Type Code 1): a) ORIGIN (Type Code 1):
ORIGIN is a well-known mandatory attribute that defines the ORIGIN is a well-known mandatory attribute that defines the
origin of the path information. The data octet can assume origin of the path information. The data octet can assume
the following values: the following values:
Value Meaning Value Meaning
0 IGP - Network Layer Reachability Information 0 IGP - Network Layer Reachability Information
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the UPDATE message has traversed the UPDATE message has traversed
The path segment length is a 1-octet long field containing The path segment length is a 1-octet long field containing
the number of ASs in the path segment value field. the number of ASs in the path segment value field.
The path segment value field contains one or more AS The path segment value field contains one or more AS
numbers, each encoded as a 2-octets long field. numbers, each encoded as a 2-octets long field.
Usage of this attribute is defined in 5.1.2. Usage of this attribute is defined in 5.1.2.
RFC DRAFT August 1992
c) NEXT_HOP (Type Code 3): c) NEXT_HOP (Type Code 3):
This is a well-known mandatory attribute that defines the IP This is a well-known mandatory attribute that defines the IP
address of the border router that should be used as the next address of the border router that should be used as the next
hop to the destinations listed in the Network Layer hop to the destinations listed in the Network Layer
Reachability field of the UPDATE message. Reachability field of the UPDATE message.
Usage of this attribute is defined in 5.1.3. Usage of this attribute is defined in 5.1.3.
d) MULTI_EXIT_DISC (Type Code 4): d) MULTI_EXIT_DISC (Type Code 4):
This is an optional non-transitive attribute that is a 1 This is an optional non-transitive attribute that is a four
octet non-negative integer. The value of this attribute may octet non-negative integer. The value of this attribute may
be used by a BGP speaker's decision process to discriminate be used by a BGP speaker's decision process to discriminate
between multiple exit points to an adjacent autonomous among multiple exit points to a neighboring autonomous
system. system.
Its usage is defined in 5.1.4. Its usage is defined in 5.1.4.
e) LOCAL_PREF (Type Code 5): e) LOCAL_PREF (Type Code 5):
LOCAL_PREF is a well-known discretionary attribute that is a LOCAL_PREF is a well-known discretionary attribute that is a
1 octet non-negative integer. It is used by a BGP speaker to four octet non-negative integer. It is used by a BGP speaker
inform other BGP speakers in its own autonomous system of to inform other BGP speakers in its own autonomous system of
the originating speaker's degree of preference for an the originating speaker's degree of preference for an
advertised route. Usage of this attribute is described in advertised route. Usage of this attribute is described in
5.1.5. 5.1.5.
f) ATOMIC_AGGREGATE (Type Code 6) f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0. It is used by a BGP speaker to inform other BGP length 0. It is used by a BGP speaker to inform other BGP
speakers that the local system selected a less specific speakers that the local system selected a less specific
route without selecting a more specific route which is route without selecting a more specific route which is
included in it. Usage of this attribute is described in included in it. Usage of this attribute is described in
5.1.6. 5.1.6.
g) AGGREGATOR (Type Code 7) g) AGGREGATOR (Type Code 7)
AGGREGATOR is an optional transitive attribute of length 2. AGGREGATOR is an optional transitive attribute of length 6.
It is used by a BGP speaker to to indicate the AS number of The attribute contains the last AS number that formed the
the last AS that formed the aggregate route. Usage of this aggregate route (encoded as 2 octets), followed by the IP
attribute is described in 5.1.7 address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). Usage of this attribute is described
in 5.1.7
Network Layer Reachability Information: Network Layer Reachability Information:
This variable length field contains a list of IP address This variable length field contains a list of IP address
RFC DRAFT August 1992
prefixes. The length in octets of the Network Layer prefixes. The length in octets of the Network Layer
Reachability Information is not encoded explicitly, but can be Reachability Information is not encoded explicitly, but can be
calculated as: calculated as:
UPDATE message Length - 23 - Total Path Attributes Length - UPDATE message Length - 23 - Total Path Attributes Length -
Unfeasible Routes Length Unfeasible Routes Length
where UPDATE message Length is the value encoded in the fixed- where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length and Unfeasible size BGP header, Total Path Attribute Length and Unfeasible
Routes Length are the values encoded in the variable part of Routes Length are the values encoded in the variable part of
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matches all IP addresses (with prefix, itself, of zero matches all IP addresses (with prefix, itself, of zero
octets). octets).
b) Prefix: b) Prefix:
The Prefix field contains IP address prefixes followed by The Prefix field contains IP address prefixes followed by
enough trailing bits to make the end of the field fall on an enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of the trailing bits is octet boundary. Note that the value of the trailing bits is
irrelevant. irrelevant.
The minimum length of the UPDATE message is 33 octets (including The minimum length of the UPDATE message is 23 octets -- 19 octets
message header). for the fixed header + 2 octets for the Unfeasible Routes Length + 2
octets for the Total Path Attribute Length (the value of Unfeasible
Routes Length is 0 and the value of Total Path Attribute Length is
0).
An UPDATE message can advertise at most one route, which may be An UPDATE message can advertise at most one route, which may be
described by several path attributes. All path attributes contained described by several path attributes. All path attributes contained
in a given UPDATE messages apply to the destinations carried in the in a given UPDATE messages apply to the destinations carried in the
RFC DRAFT August 1992
Network Layer Reachability Information field of the UPDATE message. Network Layer Reachability Information field of the UPDATE message.
An UPDATE message can list multiple routes to be withdrawn from An UPDATE message can list multiple routes to be withdrawn from
service. Each such route is identified by its destination (expressed service. Each such route is identified by its destination (expressed
as an IP prefix), which unambiguously identifies the route in the as an IP prefix), which unambiguously identifies the route in the
context of the BGP speaker - BGP speaker connection to which it has context of the BGP speaker - BGP speaker connection to which it has
been previously been advertised. been previously been advertised.
An UPDATE message may advertise only routes to be withdrawn from An UPDATE message may advertise only routes to be withdrawn from
service, in which case it will not include path attributes or Network service, in which case it will not include path attributes or Network
Layer Reachability Information. Conversely, it may advertise only a Layer Reachability Information. Conversely, it may advertise only a
feasible route, in which case the WITHDRAWN ROUTES field need not be feasible route, in which case the WITHDRAWN ROUTES field need not be
present. present.
4.4 KEEPALIVE Message Format 4.4 KEEPALIVE Message Format
BGP does not use any transport protocol-based keep-alive mechanism to BGP does not use any transport protocol-based keep-alive mechanism to
determine if peers are reachable. Instead, KEEPALIVE messages are determine if peers are reachable. Instead, KEEPALIVE messages are
exchanged between peers often enough as not to cause the hold time exchanged between peers often enough as not to cause the Hold Timer
(as advertised in the OPEN message) to expire. A reasonable maximum to expire. A reasonable maximum time between KEEPALIVE messages
time between KEEPALIVE messages would be one third of the Hold Time would be one third of the Hold Time interval. KEEPALIVE messages
interval. MUST NOT be sent more frequently than one per second. An
implementation MAY adjust the rate at which it sends KEEPALIVE
messages as a function of the Hold Time interval.
If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
messages MUST NOT be sent.
KEEPALIVE message consists of only message header and has a length of KEEPALIVE message consists of only message header and has a length of
19 octets. 19 octets.
4.5 NOTIFICATION Message Format 4.5 NOTIFICATION Message Format
A NOTIFICATION message is sent when an error condition is detected. A NOTIFICATION message is sent when an error condition is detected.
The BGP connection is closed immediately after sending it. The BGP connection is closed immediately after sending it.
In addition to the fixed-size BGP header, the NOTIFICATION message In addition to the fixed-size BGP header, the NOTIFICATION message
contains the following fields: contains the following fields:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data | | Error code | Error subcode | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RFC DRAFT August 1992
Error Code: Error Code:
This 1-octet unsigned integer indicates the type of This 1-octet unsigned integer indicates the type of
NOTIFICATION. The following Error Codes have been defined: NOTIFICATION. The following Error Codes have been defined:
Error Code Symbolic Name Reference Error Code Symbolic Name Reference
1 Message Header Error Section 6.1 1 Message Header Error Section 6.1
2 OPEN Message Error Section 6.2 2 OPEN Message Error Section 6.2
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2 - Bad Message Length. 2 - Bad Message Length.
3 - Bad Message Type. 3 - Bad Message Type.
OPEN Message Error subcodes: OPEN Message Error subcodes:
1 - Unsupported Version Number. 1 - Unsupported Version Number.
2 - Bad Peer AS. 2 - Bad Peer AS.
3 - Bad BGP Identifier. 3 - Bad BGP Identifier.
4 - Unsupported Authentication Code. 4 - Unsupported Authentication Code.
5 - Authentication Failure. 5 - Authentication Failure.
6 - Unacceptable Hold Time.
UPDATE Message Error subcodes: UPDATE Message Error subcodes:
1 - Malformed Attribute List. 1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute. 2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute. 3 - Missing Well-known Attribute.
4 - Attribute Flags Error. 4 - Attribute Flags Error.
RFC DRAFT August 1992
5 - Attribute Length Error. 5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute 6 - Invalid ORIGIN Attribute
7 - AS Routing Loop. 7 - AS Routing Loop.
8 - Invalid NEXT_HOP Attribute. 8 - Invalid NEXT_HOP Attribute.
9 - Optional Attribute Error. 9 - Optional Attribute Error.
10 - Invalid Network Field. 10 - Invalid Network Field.
11 - Malformed AS_PATH. 11 - Malformed AS_PATH.
Data: Data:
This variable-length field is used to diagnose the reason for This variable-length field is used to diagnose the reason for
the NOTIFICATION. The contents of the Data field depend upon the NOTIFICATION. The contents of the Data field depend upon
the Error Code and Error Subcode. See Section 6 below for more the Error Code and Error Subcode. See Section 6 below for more
details. details.
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Path attributes fall into four separate categories: Path attributes fall into four separate categories:
1. Well-known mandatory. 1. Well-known mandatory.
2. Well-known discretionary. 2. Well-known discretionary.
3. Optional transitive. 3. Optional transitive.
4. Optional non-transitive. 4. Optional non-transitive.
Well-known attributes must be recognized by all BGP implementations. Well-known attributes must be recognized by all BGP implementations.
Some of these attributes are mandatory and must be included in every Some of these attributes are mandatory and must be included in every
UPDATE message. Others are discretionary and may or may not be sent UPDATE message. Others are discretionary and may or may not be sent
in a particular UPDATE message. Which well-known attributes are in a particular UPDATE message.
mandatory or discretionary is noted in the table below.
All well-known attributes must be passed along (after proper All well-known attributes must be passed along (after proper
updating, if necessary) to other BGP peers. updating, if necessary) to other BGP peers.
In addition to well-known attributes, each path may contain one or In addition to well-known attributes, each path may contain one or
more optional attributes. It is not required or expected that all more optional attributes. It is not required or expected that all
RFC DRAFT August 1992
BGP implementations support all optional attributes. The handling of BGP implementations support all optional attributes. The handling of
an unrecognized optional attribute is determined by the setting of an unrecognized optional attribute is determined by the setting of
the Transitive bit in the attribute flags octet. Paths with the Transitive bit in the attribute flags octet. Paths with
unrecognized transitive optional attributes should be accepted. If a unrecognized transitive optional attributes should be accepted. If a
path with unrecognized transitive optional attribute is accepted and path with unrecognized transitive optional attribute is accepted and
passed along to other BGP peers, then the unrecognized transitive passed along to other BGP peers, then the unrecognized transitive
optional attribute of that path must be passed along with the path to optional attribute of that path must be passed along with the path to
other BGP peers with the Partial bit in the Attribute Flags octet set other BGP peers with the Partial bit in the Attribute Flags octet set
to 1. If a path with recognized transitive optional attribute is to 1. If a path with recognized transitive optional attribute is
accepted and passed along to other BGP peers and the Partial bit in accepted and passed along to other BGP peers and the Partial bit in
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The same attribute cannot appear more than once within the Path The same attribute cannot appear more than once within the Path
Attributes field of a particular UPDATE message. Attributes field of a particular UPDATE message.
5.1 Path Attribute Usage 5.1 Path Attribute Usage
The usage of each BGP path attributes is described in the following The usage of each BGP path attributes is described in the following
clauses. clauses.
5.1.1 ORIGIN 5.1.1 ORIGIN
ORIGIN is a well-known mandatory attribute. It shall be recognized ORIGIN is a well-known mandatory attribute. The ORIGIN attribute
shall be generated by the autonomous system that originates the
RFC DRAFT August 1992 associated routing information. It shall be included in the UPDATE
messages of all BGP speakers that choose to propagate this
upon receipt by all BGP speakers. It shall be included in each UPDATE information to other BGP speakers.
message that includes Network Layer Reachability Information.
The ORIGIN attribute shall be generated by the autonomous system that
originates the associated routing information. It shall be included
in the UPDATE messages of all BGP speakers that choose to propagate
this information to other BGP speakers.
5.1.2 AS_PATH 5.1.2 AS_PATH
AS_PATH is a well-known mandatory attribute. It shall be presented in AS_PATH is a well-known mandatory attribute. This attribute
every UPDATE message and shall be recognized upon receipt by all BGP identifies the autonomous systems through which routing information
speakers. This attribute identifies the autonomous systems through carried in this UPDATE message has passed. The components of this
which routing information carried in this UPDATE message has passed. list can be AS_SETs or AS_SEQUENCEs.
The components of this list can be AS_SETs or AS_SEQUENCEs.
When a BGP speaker propagates a route which it has learned from When a BGP speaker propagates a route which it has learned from
another BGP speaker's UPDATE message, it shall modify the route's another BGP speaker's UPDATE message, it shall modify the route's
AS_PATH attribute based on the location of the BGP speaker to which AS_PATH attribute based on the location of the BGP speaker to which
the route will be sent: the route will be sent:
a) When a given BGP speaker advertises the route to another BGP a) When a given BGP speaker advertises the route to another BGP
speaker located in its own autonomous system, the advertising speaker located in its own autonomous system, the advertising
speaker shall not modify the AS_PATH attribute associated with the speaker shall not modify the AS_PATH attribute associated with the
route. route.
b) When a given BGP speaker advertises the route to a BGP speaker b) When a given BGP speaker advertises the route to a BGP speaker
located in an adjacent autonomous system, then the advertising located in a neighboring autonomous system, then the advertising
speaker shall update the AS_PATH attribute as follows: speaker shall update the AS_PATH attribute as follows:
1) if the first path segment of the AS_PATH is of type 1) if the first path segment of the AS_PATH is of type
AS_SEQUENCE, the local system shall prepend its own AS number AS_SEQUENCE, the local system shall prepend its own AS number
as the last element of the sequence (put it in the leftmost as the last element of the sequence (put it in the leftmost
position) position).
2) if the first path segment of the AS_PATH is of type AS_SET, 2) if the first path segment of the AS_PATH is of type AS_SET,
the local system shall prepend a new path segment of type the local system shall prepend a new path segment of type
AS_SEQUENCE to the AS_PATH, including its own AS number in that AS_SEQUENCE to the AS_PATH, including its own AS number in that
segment. segment.
When a BGP speaker originates a route then: When a BGP speaker originates a route then:
a) the originating speaker shall include its own AS number in a) the originating speaker shall include its own AS number in
the AS_PATH attribute of all UPDATE messages sent to BGP the AS_PATH attribute of all UPDATE messages sent to BGP
speakers located in adjacent autonomous systems. (In this case, speakers located in neighboring autonomous systems. (In this
case, the AS number of the originating speaker's autonomous
RFC DRAFT August 1992 system will be the only entry in the AS_PATH attribute).
the AS number of the originating speaker's autonomous system
will be the only entry in the AS_PATH attribute).
b) the originating speaker shall include an empty AS_PATH b) the originating speaker shall include an empty AS_PATH
attribute in all UPDATE messages sent to BGP speakers located attribute in all UPDATE messages sent to BGP speakers located
in its own autonomous system. (An empty AS_PATH attribute is in its own autonomous system. (An empty AS_PATH attribute is
one whose length field contains the value zero). one whose length field contains the value zero).
5.1.3 NEXT_HOP 5.1.3 NEXT_HOP
The NEXT_HOP path attribute defines the IP address of the border The NEXT_HOP path attribute defines the IP address of the border
router that should be used as the next hop to the networks listed in router that should be used as the next hop to the networks listed in
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with the IP address of this border router (as specified in the with the IP address of this border router (as specified in the
NEXT_HOP path attribute) shares a common subnet with both the local NEXT_HOP path attribute) shares a common subnet with both the local
and remote BGP speakers. A BGP speaker can advertise any external and remote BGP speakers. A BGP speaker can advertise any external
border router as the next hop, provided that the IP address of this border router as the next hop, provided that the IP address of this
border router was learned from one of the BGP speaker's peers, and border router was learned from one of the BGP speaker's peers, and
the interface associated with the IP address of this border router the interface associated with the IP address of this border router
(as specified in the NEXT_HOP path attribute) shares a common subnet (as specified in the NEXT_HOP path attribute) shares a common subnet
with the local and remote BGP speakers. A BGP speaker needs to be with the local and remote BGP speakers. A BGP speaker needs to be
able to support disabling advertisement of external border routers. able to support disabling advertisement of external border routers.
A BGP speaker must never advertise an address of a neighbor to that A BGP speaker must never advertise an address of a peer to that peer
neighbor as a NEXT_HOP, for a route that the speaker is originating. as a NEXT_HOP, for a route that the speaker is originating. A BGP
A BGP speaker must never install a route with itself as the next hop. speaker must never install a route with itself as the next hop.
When a BGP speaker advertises the route to a BGP speaker located in When a BGP speaker advertises the route to a BGP speaker located in
its own autonomous system, the advertising speaker shall not modify its own autonomous system, the advertising speaker shall not modify
the NEXT_HOP attribute associated with the route. When a BGP speaker the NEXT_HOP attribute associated with the route. When a BGP speaker
receives the route via an internal link, it may use that NEXT_HOP if receives the route via an internal link, it may forward packets to
the address contained in the attribute is on a common subnet with the the NEXT_HOP address if the address contained in the attribute is on
local and remote BGP speakers. The BGP speaker may also use the a common subnet with the local and remote BGP speakers.
NEXT_HOP address if the IGP does not contain a route for the
destination.
5.1.4 MULTI_EXIT_DISC 5.1.4 MULTI_EXIT_DISC
The MULTI_EXIT_DISC attribute may be used on external (inter-AS) The MULTI_EXIT_DISC attribute may be used on external (inter-AS)
links to discriminate between multiple exit or entry points to the links to discriminate among multiple exit or entry points to the same
same neighboring AS. The value of the MULTI_EXIT_DISC attribute is a neighboring AS. The value of the MULTI_EXIT_DISC attribute is a four
octet unsigned number which is called a metric. All other factors
RFC DRAFT August 1992
1-octet unsigned number which is called a metric. All other factors
being equal, the exit or entry point with lower metric should be being equal, the exit or entry point with lower metric should be
preferred. If received over external links, the MULTI_ EXIT_DISC preferred. If received over external links, the MULTI_EXIT_DISC
attribute may be propagated over internal links to other BGP speakers attribute may be propagated over internal links to other BGP speakers
within the same AS. The MULTI_EXIT_DISC attribute is never within the same AS. The MULTI_EXIT_DISC attribute is never
propagated to other BGP speakers in neighboring AS's. propagated to other BGP speakers in neighboring AS's.
5.1.5 LOCAL_PREF 5.1.5 LOCAL_PREF
LOCAL_PREF is a well-known discretionary attribute that shall be LOCAL_PREF is a well-known discretionary attribute that shall be
included in all UPDATE messages that a given BGP speaker sends to the included in all UPDATE messages that a given BGP speaker sends to the
other BGP speakers located in its own autonomous system. A BGP other BGP speakers located in its own autonomous system. A BGP
speaker shall calculate the degree of preference for each external speaker shall calculate the degree of preference for each external
route and include the degree of preference when advertising a route route and include the degree of preference when advertising a route
to its internal neighbors. The lower degree of preference should be to its internal peers. The higher degree of preference should be
preferred. A BGP speaker shall use the degree of preference learned preferred. A BGP speaker shall use the degree of preference learned
via LOCAL_PREF in its decision process (see section 9.1.1). via LOCAL_PREF in its decision process (see section 9.1.1).
A BGP speaker shall not include this attribute in UPDATE messages A BGP speaker shall not include this attribute in UPDATE messages
that it sends to BGP speakers located in an adjacent autonomous that it sends to BGP speakers located in a neighboring autonomous
system. It is contained in an UPDATE message that is received from a system. If it is contained in an UPDATE message that is received from
BGP speaker which is not located in the same autonomous system as the a BGP speaker which is not located in the same autonomous system as
receiving speaker, then this attribute shall be ignored by the the receiving speaker, then this attribute shall be ignored by the
receiving speaker. receiving speaker.
5.1.6 ATOMIC_AGGREGATE 5.1.6 ATOMIC_AGGREGATE
ATOMIC_AGGREGATE is a well-known discretionary attribute. If a BGP ATOMIC_AGGREGATE is a well-known discretionary attribute. If a BGP
speaker, when presented with a set of overlapping routes from one of speaker, when presented with a set of overlapping routes from one of
its peers (see 9.1.4), selects the less specific route without its peers (see 9.1.4), selects the less specific route without
selecting the more specific one, then the local system shall attach selecting the more specific one, then the local system shall attach
the ATOMIC_AGGREGATE attribute to the route when propagating it to the ATOMIC_AGGREGATE attribute to the route when propagating it to
other BGP speakers (if that attribute is not already present in the other BGP speakers (if that attribute is not already present in the
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from the route when propagating it to other speakers. A BGP speaker from the route when propagating it to other speakers. A BGP speaker
that receives a route with the ATOMIC_AGGREGATE attribute shall not that receives a route with the ATOMIC_AGGREGATE attribute shall not
make any NLRI of that route more specific (as defined in 9.1.4) when make any NLRI of that route more specific (as defined in 9.1.4) when
advertising this route to other BGP speakers. A BGP speaker that advertising this route to other BGP speakers. A BGP speaker that
receives a route with the ATOMIC_AGGREGATE attribute needs to be receives a route with the ATOMIC_AGGREGATE attribute needs to be
cognizant of the fact that the actual path to destinations, as cognizant of the fact that the actual path to destinations, as
specified in the NLRI of the route, while having the loop-free specified in the NLRI of the route, while having the loop-free
property, may traverse ASs that are not listed in the AS_PATH property, may traverse ASs that are not listed in the AS_PATH
attribute. attribute.
RFC DRAFT August 1992
5.1.7 AGGREGATOR 5.1.7 AGGREGATOR
AGGREGATOR is an optional transitive attribute which may be included AGGREGATOR is an optional transitive attribute which may be included
in updates which are formed by aggregation (see Section 9.2.4.2). A in updates which are formed by aggregation (see Section 9.2.4.2). A
BGP speaker which performs route aggregation may add the AGGREGATOR BGP speaker which performs route aggregation may add the AGGREGATOR
attribute which shall contain its own AS number. attribute which shall contain its own AS number and IP address.
6. BGP Error Handling. 6. BGP Error Handling.
This section describes actions to be taken when errors are detected This section describes actions to be taken when errors are detected
while processing BGP messages. while processing BGP messages.
When any of the conditions described here are detected, a When any of the conditions described here are detected, a
NOTIFICATION message with the indicated Error Code, Error Subcode, NOTIFICATION message with the indicated Error Code, Error Subcode,
and Data fields is sent, and the BGP connection is closed. If no and Data fields is sent, and the BGP connection is closed. If no
Error Subcode is specified, then a zero must be used. Error Subcode is specified, then a zero must be used.
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The expected value of the Marker field of the message header is all The expected value of the Marker field of the message header is all
ones if the message type is OPEN. The expected value of the Marker ones if the message type is OPEN. The expected value of the Marker
field for all other types of BGP messages determined based on the field for all other types of BGP messages determined based on the
Authentication Code in the BGP OPEN message and the actual Authentication Code in the BGP OPEN message and the actual
authentication mechanism (if the Authentication Code in the BGP OPEN authentication mechanism (if the Authentication Code in the BGP OPEN
message is non-zero). If the Marker field of the message header is message is non-zero). If the Marker field of the message header is
not the expected one, then a synchronization error has occurred and not the expected one, then a synchronization error has occurred and
the Error Subcode is set to Connection Not Synchronized. the Error Subcode is set to Connection Not Synchronized.
RFC DRAFT August 1992
If the Length field of the message header is less than 19 or greater If the Length field of the message header is less than 19 or greater
than 4096, or if the Length field of an OPEN message is less than than 4096, or if the Length field of an OPEN message is less than
the minimum length of the OPEN message, or if the Length field of an the minimum length of the OPEN message, or if the Length field of an
UPDATE message is less than the minimum length of the UPDATE message, UPDATE message is less than the minimum length of the UPDATE message,
or if the Length field of a KEEPALIVE message is not equal to 19, or or if the Length field of a KEEPALIVE message is not equal to 19, or
if the Length field of a NOTIFICATION message is less than the if the Length field of a NOTIFICATION message is less than the
minimum length of the NOTIFICATION message, then the Error Subcode is minimum length of the NOTIFICATION message, then the Error Subcode is
set to Bad Message Length. The Data field contains the erroneous set to Bad Message Length. The Data field contains the erroneous
Length field. Length field.
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Unsupported Version Number. The Data field is a 2-octet unsigned Unsupported Version Number. The Data field is a 2-octet unsigned
integer, which indicates the largest locally supported version number integer, which indicates the largest locally supported version number
less than the version the remote BGP peer bid (as indicated in the less than the version the remote BGP peer bid (as indicated in the
received OPEN message). received OPEN message).
If the Autonomous System field of the OPEN message is unacceptable, If the Autonomous System field of the OPEN message is unacceptable,
then the Error Subcode is set to Bad Peer AS. The determination of then the Error Subcode is set to Bad Peer AS. The determination of
acceptable Autonomous System numbers is outside the scope of this acceptable Autonomous System numbers is outside the scope of this
protocol. protocol.
If the Hold Time field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to Unacceptable Hold Time. An
implementation MUST reject Hold Time values of one or two seconds.
An implementation MAY reject any proposed Hold Time. An
implementation which accepts a Hold Time MUST use the negotiated
value for the Hold Time.
If the BGP Identifier field of the OPEN message is syntactically If the BGP Identifier field of the OPEN message is syntactically
incorrect, then the Error Subcode is set to Bad BGP Identifier. incorrect, then the Error Subcode is set to Bad BGP Identifier.
Syntactic correctness means that the BGP Identifier field represents Syntactic correctness means that the BGP Identifier field represents
a valid IP host address. a valid IP host address.
If the Authentication Code of the OPEN message is not recognized, If the Authentication Code of the OPEN message is not recognized,
then the Error Subcode is set to Unsupported Authentication Code. If then the Error Subcode is set to Unsupported Authentication Code. If
the Authentication Code is zero, then the Authentication Data must be the Authentication Code is zero, then the Authentication Data must be
of zero length. Otherwise, the Error Subcode is set to of zero length. Otherwise, the Error Subcode is set to
Authentication Failure. Authentication Failure.
If the Authentication Code is non-zero, then the corresponding If the Authentication Code is non-zero, then the corresponding
authentication procedure is invoked. If the authentication procedure authentication procedure is invoked. If the authentication procedure
RFC DRAFT August 1992
(based on Authentication Code and Authentication Data) fails, then (based on Authentication Code and Authentication Data) fails, then
the Error Subcode is set to Authentication Failure. the Error Subcode is set to Authentication Failure.
6.3 UPDATE message error handling. 6.3 UPDATE message error handling.
All errors detected while processing the UPDATE message are indicated All errors detected while processing the UPDATE message are indicated
by sending the NOTIFICATION message with Error Code UPDATE Message by sending the NOTIFICATION message with Error Code UPDATE Message
Error. The error subcode elaborates on the specific nature of the Error. The error subcode elaborates on the specific nature of the
error. error.
Error checking of an UPDATE message begins by examining the path Error checking of an UPDATE message begins by examining the path
attributes. If the Total Attribute Length is too large (i.e., if attributes. If the Unfeasible Routes Length or Total Attribute
Total Attribute Length + 21 exceeds the message Length), or if the Length is too large (i.e., if Unfeasible Routes Length + Total
(non-negative integer) Number of Network fields cannot be computed as Attribute Length + 23 exceeds the message Length), then the Error
in Section 4.3, then the Error Subcode is set to Malformed Attribute Subcode is set to Malformed Attribute List.
List.
If any recognized attribute has Attribute Flags that conflict with If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode is set to Attribute the Attribute Type Code, then the Error Subcode is set to Attribute
Flags Error. The Data field contains the erroneous attribute (type, Flags Error. The Data field contains the erroneous attribute (type,
length and value). length and value).
If any recognized attribute has Attribute Length that conflicts with If any recognized attribute has Attribute Length that conflicts with
the expected length (based on the attribute type code), then the the expected length (based on the attribute type code), then the
Error Subcode is set to Attribute Length Error. The Data field Error Subcode is set to Attribute Length Error. The Data field
contains the erroneous attribute (type, length and value). contains the erroneous attribute (type, length and value).
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If any of the mandatory well-known attributes are not recognized, If any of the mandatory well-known attributes are not recognized,
then the Error Subcode is set to Unrecognized Well-known Attribute. then the Error Subcode is set to Unrecognized Well-known Attribute.
The Data field contains the unrecognized attribute (type, length and The Data field contains the unrecognized attribute (type, length and
value). value).
If the ORIGIN attribute has an undefined value, then the Error If the ORIGIN attribute has an undefined value, then the Error
Subcode is set to Invalid Origin Attribute. The Data field contains Subcode is set to Invalid Origin Attribute. The Data field contains
the unrecognized attribute (type, length and value). the unrecognized attribute (type, length and value).
If the NEXT_HOP attribute field is syntactically or semantically If the NEXT_HOP attribute field is syntactically incorrect, then the
incorrect, then the Error Subcode is set to Invalid NEXT_HOP Error Subcode is set to Invalid NEXT_HOP Attribute. The Data field
Attribute. contains the incorrect attribute (type, length and value). Syntactic
correctness means that the NEXT_HOP attribute represents a valid IP
The Data field contains the incorrect attribute (type, length and host address. Semantic correctness applies only to the external BGP
links. It means that the interface associated with the IP address, as
RFC DRAFT August 1992 specified in the NEXT_HOP attribute, shares a common subnet with the
receiving BGP speaker and is not the IP address of the receiving BGP
value). Syntactic correctness means that the NEXT_HOP attribute speaker. If the NEXT_HOP attribute is semantically incorrect, the
represents a valid IP host address. Semantic correctness applies error should be logged, and the the route should be ignored. In this
only to the external BGP links. It means that the interface case, no NOTIFICATION message should be sent.
associated with the IP address, as specified in the NEXT_HOP
attribute, shares a common subnet with the receiving BGP speaker and
is not the IP address of the receiving BGP speaker.
The AS_PATH attribute is checked for syntactic correctness. If the The AS_PATH attribute is checked for syntactic correctness. If the
path is syntactically incorrect, then the Error Subcode is set to path is syntactically incorrect, then the Error Subcode is set to
Malformed AS_PATH. Malformed AS_PATH.
The AS route specified by the AS_PATH attribute is checked for AS
loops. AS loop detection is done by scanning the full AS route (as
specified in the AS_PATH attribute) and checking that each AS occurs
at most once. If a loop is detected, then the Error Subcode is set
to AS Routing Loop. The Data field contains the incorrect attribute
(type, length and value).
If an optional attribute is recognized, then the value of this If an optional attribute is recognized, then the value of this
attribute is checked. If an error is detected, the attribute is attribute is checked. If an error is detected, the attribute is
discarded, and the Error Subcode is set to Optional Attribute Error. discarded, and the Error Subcode is set to Optional Attribute Error.
The Data field contains the attribute (type, length and value). The Data field contains the attribute (type, length and value).
If any attribute appears more than once in the UPDATE message, then If any attribute appears more than once in the UPDATE message, then
the Error Subcode is set to Malformed Attribute List. the Error Subcode is set to Malformed Attribute List.
Each Network field in the UPDATE message is checked for syntactic The NLRI field in the UPDATE message is checked for syntactic
validity. If the Network field is syntactically incorrect, or validity. If the field is syntactically incorrect, then the Error
contains a subnet or a host address, then the Error Subcode is set to Subcode is set to Invalid Network Field.
Invalid Network Field.
6.4 NOTIFICATION message error handling. 6.4 NOTIFICATION message error handling.
If a peer sends a NOTIFICATION message, and there is an error in that If a peer sends a NOTIFICATION message, and there is an error in that
message, there is unfortunately no means of reporting this error via message, there is unfortunately no means of reporting this error via
a subsequent NOTIFICATION message. Any such error, such as an a subsequent NOTIFICATION message. Any such error, such as an
unrecognized Error Code or Error Subcode, should be noticed, logged unrecognized Error Code or Error Subcode, should be noticed, logged
locally, and brought to the attention of the administration of the locally, and brought to the attention of the administration of the
peer. The means to do this, however, lies outside the scope of this peer. The means to do this, however, lies outside the scope of this
document. document.
6.5 Hold Timer Expired error handling. 6.5 Hold Timer Expired error handling.
If a system does not receive successive KEEPALIVE and/or UPDATE If a system does not receive successive KEEPALIVE and/or UPDATE
RFC DRAFT August 1992
and/or NOTIFICATION messages within the period specified in the Hold and/or NOTIFICATION messages within the period specified in the Hold
Time field of the OPEN message, then the NOTIFICATION message with Time field of the OPEN message, then the NOTIFICATION message with
Hold Timer Expired Error Code must be sent and the BGP connection Hold Timer Expired Error Code must be sent and the BGP connection
closed. closed.
6.6 Finite State Machine error handling. 6.6 Finite State Machine error handling.
Any error detected by the BGP Finite State Machine (e.g., receipt of Any error detected by the BGP Finite State Machine (e.g., receipt of
an unexpected event) is indicated by sending the NOTIFICATION message an unexpected event) is indicated by sending the NOTIFICATION message
with Error Code Finite State Machine Error. with Error Code Finite State Machine Error.
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Based on the value of the BGP Identifier a convention is established Based on the value of the BGP Identifier a convention is established
for detecting which BGP connection is to be preserved when a for detecting which BGP connection is to be preserved when a
collision does occur. The convention is to compare the BGP collision does occur. The convention is to compare the BGP
Identifiers of the peers involved in the collision and to retain only Identifiers of the peers involved in the collision and to retain only
the connection initiated by the BGP speaker with the higher-valued the connection initiated by the BGP speaker with the higher-valued
BGP Identifier. BGP Identifier.
Upon receipt of an OPEN message, the local system must examine all of Upon receipt of an OPEN message, the local system must examine all of
its connections that are in the OpenConfirm state. A BGP speaker may its connections that are in the OpenConfirm state. A BGP speaker may
also examine connections in an OpenSent state if it knows the BGP also examine connections in an OpenSent state if it knows the BGP
Identifier of the neighbor by means outside of the protocol. If Identifier of the peer by means outside of the protocol. If among
among these connections there is a connection to a remote BGP speaker these connections there is a connection to a remote BGP speaker whose
whose BGP Identifier equals the one in the OPEN message, then the BGP Identifier equals the one in the OPEN message, then the local
local system performs the following collision resolution procedure: system performs the following collision resolution procedure:
RFC DRAFT August 1992
1. The BGP Identifier of the local system is compared to the BGP 1. The BGP Identifier of the local system is compared to the BGP
Identifier of the remote system (as specified in the OPEN Identifier of the remote system (as specified in the OPEN
message). message).
2. If the value of the local BGP Identifier is less than the 2. If the value of the local BGP Identifier is less than the
remote one, the local system closes BGP connection that already remote one, the local system closes BGP connection that already
exists (the one that is already in the OpenConfirm state), and exists (the one that is already in the OpenConfirm state), and
accepts BGP connection initiated by the remote system. accepts BGP connection initiated by the remote system.
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Code OPEN Message Error, and an Error Subcode Unsupported Version Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available the version number it Number, then the BGP speaker has available the version number it
tried, the version number its peer tried, the version number passed tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers that by its peer in the NOTIFICATION message, and the version numbers that
it supports. If the two peers do support one or more common it supports. If the two peers do support one or more common
versions, then this will allow them to rapidly determine the highest versions, then this will allow them to rapidly determine the highest
common version. In order to support BGP version negotiation, future common version. In order to support BGP version negotiation, future
versions of BGP must retain the format of the OPEN and NOTIFICATION versions of BGP must retain the format of the OPEN and NOTIFICATION
messages. messages.
RFC DRAFT August 1992
8. BGP Finite State machine. 8. BGP Finite State machine.
This section specifies BGP operation in terms of a Finite State This section specifies BGP operation in terms of a Finite State
Machine (FSM). Following is a brief summary and overview of BGP Machine (FSM). Following is a brief summary and overview of BGP
operations by state as determined by this FSM. A condensed version operations by state as determined by this FSM. A condensed version
of the BGP FSM is found in Appendix 1. of the BGP FSM is found in Appendix 1.
Initially BGP is in the Idle state. Initially BGP is in the Idle state.
Idle state: Idle state:
In this state BGP refuses all incoming BGP connections. No In this state BGP refuses all incoming BGP connections. No
resources are allocated to the BGP neighbor. In response to resources are allocated to the peer. In response to the Start
the Start event (initiated by either system or operator) the event (initiated by either system or operator) the local system
local system initializes all BGP resources, starts the initializes all BGP resources, starts the ConnectRetry timer,
ConnectRetry timer, initiates a transport connection to other initiates a transport connection to other BGP peer, while
BGP peer, while listening for connection that may be initiated listening for connection that may be initiated by the remote
by the remote BGP peer, and changes its state to Connect. The BGP peer, and changes its state to Connect. The exact value of
exact value of the ConnectRetry timer is a local matter, but the ConnectRetry timer is a local matter, but should be
should be sufficiently large to allow TCP initialization. sufficiently large to allow TCP initialization.
If a BGP speaker detects an error, it shuts down the connection If a BGP speaker detects an error, it shuts down the connection
and changes its state to Idle. Getting out of the Idle state and changes its state to Idle. Getting out of the Idle state
requires generation of the Start event. If such an event is requires generation of the Start event. If such an event is
generated automatically, then persistent BGP errors may result generated automatically, then persistent BGP errors may result
in persistent flapping of the speaker. To avoid such a in persistent flapping of the speaker. To avoid such a
condition it is recommended that Start events should not be condition it is recommended that Start events should not be
generated immediately for a peer that was previously generated immediately for a peer that was previously
transitioned to Idle due to an error. For a peer that was transitioned to Idle due to an error. For a peer that was
previously transitioned to Idle due to an error, the time previously transitioned to Idle due to an error, the time
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Connect state: Connect state:
In this state BGP is waiting for the transport protocol In this state BGP is waiting for the transport protocol
connection to be completed. connection to be completed.
If the transport protocol connection succeeds, the local system If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, and changes its state to OpenSent. an OPEN message to its peer, and changes its state to OpenSent.
If the transport protocol connect fails (e.g., retransmission If the transport protocol connect fails (e.g., retransmission
RFC DRAFT August 1992
timeout), the local system restarts the ConnectRetry timer, timeout), the local system restarts the ConnectRetry timer,
continues to listen for a connection that may be initiated by continues to listen for a connection that may be initiated by
the remote BGP peer, and changes its state to Active state. the remote BGP peer, and changes its state to Active state.
In response to the ConnectRetry timer expired event, the local In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and connection that may be initiated by the remote BGP peer, and
stays in the Connect state. stays in the Connect state.
Start event is ignored in the Active state. Start event is ignored in the Active state.
In response to any other event (initiated by either system or In response to any other event (initiated by either system or
operator), the local system releases all BGP resources operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle. associated with this connection and changes its state to Idle.
Active state: Active state:
In this state BGP is trying to acquire a BGP neighbor by In this state BGP is trying to acquire a peer by initiating a
initiating a transport protocol connection. transport protocol connection.
If the transport protocol connection succeeds, the local system If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, sets its hold timer to a large an OPEN message to its peer, sets its Hold Timer to a large
value, and changes its state to OpenSent. value, and changes its state to OpenSent. A Hold Timer value
of 4 minutes is suggested.
In response to the ConnectRetry timer expired event, the local In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and connection that may be initiated by the remote BGP peer, and
changes its state to Connect. changes its state to Connect.
If the local system detects that a remote peer is trying to If the local system detects that a remote peer is trying to
establish BGP connection to it, and the IP address of the establish BGP connection to it, and the IP address of the
remote peer is not an expected one, the local system restarts remote peer is not an expected one, the local system restarts
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Start event is ignored in the Active state. Start event is ignored in the Active state.
In response to any other event (initiated by either system or In response to any other event (initiated by either system or
operator), the local system releases all BGP resources operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle. associated with this connection and changes its state to Idle.
OpenSent state: OpenSent state:
In this state BGP waits for an OPEN message from its peer. In this state BGP waits for an OPEN message from its peer.
RFC DRAFT August 1992
When an OPEN message is received, all fields are checked for When an OPEN message is received, all fields are checked for
correctness. If the BGP message header checking or OPEN correctness. If the BGP message header checking or OPEN
message checking detects an error (see Section 6.2), or a message checking detects an error (see Section 6.2), or a
connection collision (see Section 6.8) the local system sends a connection collision (see Section 6.8) the local system sends a
NOTIFICATION message and changes its state to Idle. NOTIFICATION message and changes its state to Idle.
If there are no errors in the OPEN message, BGP sends a If there are no errors in the OPEN message, BGP sends a
KEEPALIVE message and sets a KeepAlive timer. The hold timer, KEEPALIVE message and sets a KeepAlive timer. The Hold Timer,
which was originally set to an arbitrary large value (see which was originally set to a large value (see above), is
above), is replaced with the value indicated in the OPEN replaced with the negotiated Hold Time value (see section 4.2).
message. If the value of the Autonomous System field is the If the negotiated Hold Time value is zero, then the Hold Time
same as our own, then the connection is "internal" connection; timer and KeepAlive timers are not started. If the value of
the Autonomous System field is the same as the local Autonomous
System number, then the connection is an "internal" connection;
otherwise, it is "external". (This will effect UPDATE otherwise, it is "external". (This will effect UPDATE
processing as described below.) Finally, the state is changed processing as described below.) Finally, the state is changed
to OpenConfirm. to OpenConfirm.
If a disconnect notification is received from the underlying If a disconnect notification is received from the underlying
transport protocol, the local system closes the BGP connection, transport protocol, the local system closes the BGP connection,
restarts the ConnectRetry timer, while continue listening for restarts the ConnectRetry timer, while continue listening for
connection that may be initiated by the remote BGP peer, and connection that may be initiated by the remote BGP peer, and
goes into the Active state. goes into the Active state.
If the hold time expires, the local system sends NOTIFICATION If the Hold Timer expires, the local system sends NOTIFICATION
message with error code Hold Timer Expired and changes its message with error code Hold Timer Expired and changes its
state to Idle. state to Idle.
In response to the Stop event (initiated by either system or In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle. Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenSent state. Start event is ignored in the OpenSent state.
In response to any other event the local system sends In response to any other event the local system sends
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resources associated with that connection. resources associated with that connection.
OpenConfirm state: OpenConfirm state:
In this state BGP waits for a KEEPALIVE or NOTIFICATION In this state BGP waits for a KEEPALIVE or NOTIFICATION
message. message.
If the local system receives a KEEPALIVE message, it changes If the local system receives a KEEPALIVE message, it changes
its state to Established. its state to Established.
RFC DRAFT August 1992 If the Hold Timer expires before a KEEPALIVE message is
If the hold timer expires before a KEEPALIVE message is
received, the local system sends NOTIFICATION message with received, the local system sends NOTIFICATION message with
error code Hold Timer expired and changes its state to Idle. error code Hold Timer Expired and changes its state to Idle.
If the local system receives a NOTIFICATION message, it changes If the local system receives a NOTIFICATION message, it changes
its state to Idle. its state to Idle.
If the KeepAlive timer expires, the local system sends a If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer. KEEPALIVE message and restarts its KeepAlive timer.
If a disconnect notification is received from the underlying If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle. transport protocol, the local system changes its state to Idle.
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Whenever BGP changes its state from OpenConfirm to Idle, it Whenever BGP changes its state from OpenConfirm to Idle, it
closes the BGP (and transport-level) connection and releases closes the BGP (and transport-level) connection and releases
all resources associated with that connection. all resources associated with that connection.
Established state: Established state:
In the Established state BGP can exchange UPDATE, NOTIFICATION, In the Established state BGP can exchange UPDATE, NOTIFICATION,
and KEEPALIVE messages with its peer. and KEEPALIVE messages with its peer.
If the local system receives an UPDATE or KEEPALIVE message, it If the local system receives an UPDATE or KEEPALIVE message, it
restarts its Holdtime timer. restarts its Hold Timer, if the negotiated Hold Time value is
non-zero.
If the local system receives a NOTIFICATION message, it changes If the local system receives a NOTIFICATION message, it changes
its state to Idle. its state to Idle.
If the local system receives an UPDATE message and the UPDATE If the local system receives an UPDATE message and the UPDATE
message error handling procedure (see Section 6.3) detects an message error handling procedure (see Section 6.3) detects an
error, the local system sends a NOTIFICATION message and error, the local system sends a NOTIFICATION message and
changes its state to Idle. changes its state to Idle.
If a disconnect notification is received from the underlying If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle. transport protocol, the local system changes its state to Idle.
If the Holdtime timer expires, the local system sends a If the Hold Timer expires, the local system sends a
NOTIFICATION message with Error Code Hold Timer Expired and NOTIFICATION message with Error Code Hold Timer Expired and
RFC DRAFT August 1992
changes its state to Idle. changes its state to Idle.
If the KeepAlive timer expires, the local system sends a If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer. KEEPALIVE message and restarts its KeepAlive timer.
Each time the local system sends a KEEPALIVE or UPDATE message, Each time the local system sends a KEEPALIVE or UPDATE message,
it restarts its KeepAlive timer. it restarts its KeepAlive timer, unless the negotiated Hold
Time value is zero.
In response to the Stop event (initiated by either system or In response to the Stop event (initiated by either system or
operator), the local system sends a NOTIFICATION message with operator), the local system sends a NOTIFICATION message with
Error Code Cease and changes its state to Idle. Error Code Cease and changes its state to Idle.
Start event is ignored in the Established state. Start event is ignored in the Established state.
In response to any other event, the local system sends In response to any other event, the local system sends
NOTIFICATION message with Error Code Finite State Machine Error NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle. and changes its state to Idle.
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unrecognized, the Partial bit (the third high-order bit) in the unrecognized, the Partial bit (the third high-order bit) in the
attribute flags octet is set to 1, and the attribute is retained for attribute flags octet is set to 1, and the attribute is retained for
propagation to other BGP speakers. propagation to other BGP speakers.
If an optional attribute is recognized, and has a valid value, then, If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is processed depending on the type of the optional attribute, it is processed
locally, retained, and updated, if necessary, for possible locally, retained, and updated, if necessary, for possible
propagation to other BGP speakers. propagation to other BGP speakers.
If the UPDATE message contains a non-empty WITHDRAWN ROUTES field, If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
the previously advertised routes whose destinations (expressed as IP the previously advertised routes whose destinations (expressed as IP
prefixes) contained in this field shall be removed from the Adj-RIB- prefixes) contained in this field shall be removed from the Adj-RIB-
In. This BGP speaker shall run its Decision Process since the In. This BGP speaker shall run its Decision Process since the
previously advertised route is not longer available for use. previously advertised route is not longer available for use.
RFC DRAFT August 1992
If the UPDATE message contains a feasible route, it shall be placed If the UPDATE message contains a feasible route, it shall be placed
in the appropriate Adj-RIB-In, and the following additional actions in the appropriate Adj-RIB-In, and the following additional actions
shall be taken: shall be taken:
i) If its Network Layer Reachability Information (NLRI) is identical i) If its Network Layer Reachability Information (NLRI) is identical
to the one of a route currently stored in the Adj-RIB-In, then the to the one of a route currently stored in the Adj-RIB-In, then the
new route shall replace the older route in the Adj-RIB-In, thus new route shall replace the older route in the Adj-RIB-In, thus
implicitly withdrawing the older route from service. The BGP speaker implicitly withdrawing the older route from service. The BGP speaker
shall run its Decision Process since the older route is no longer shall run its Decision Process since the older route is no longer
available for use. available for use.
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v) If the new route is an overlapping route that is less specific v) If the new route is an overlapping route that is less specific
(see 9.1.4) than an earlier route contained in the Adj-RIB-In, the (see 9.1.4) than an earlier route contained in the Adj-RIB-In, the
BGP speaker shall run its Decision Process on the set of destinations BGP speaker shall run its Decision Process on the set of destinations
described only by the less specific route. described only by the less specific route.
9.1 Decision Process 9.1 Decision Process
The Decision Process selects routes for subsequent advertisement by The Decision Process selects routes for subsequent advertisement by
applying the policies in the local Policy Information Base (PIB) to applying the policies in the local Policy Information Base (PIB) to
the routes stored in its Adj-RIB-In. The output of the Decision the routes stored in its Adj-RIB-In. The output of the Decision
Process is the set of routes that will be advertised to adjacent BGP Process is the set of routes that will be advertised to all peers;
speakers; the selected routes will be stored in the local speaker's the selected routes will be stored in the local speaker's Adj-RIB-
Adj-RIB-Out. Out.
The selection process is formalized by defining a function that takes The selection process is formalized by defining a function that takes
the attribute of a given route as an argument and returns a non- the attribute of a given route as an argument and returns a non-
negative integer denoting the degree of preference for the route. negative integer denoting the degree of preference for the route.
The function that calculates the degree of preference for a given The function that calculates the degree of preference for a given
route shall not use as its inputs any of the following: the existence route shall not use as its inputs any of the following: the existence
of other routes, the non-existence of other routes, or the path of other routes, the non-existence of other routes, or the path
RFC DRAFT August 1992
attributes of other routes. Route selection then consists of attributes of other routes. Route selection then consists of
individual application of the degree of preference function to each individual application of the degree of preference function to each
feasible route, followed by the choice of the one with the highest feasible route, followed by the choice of the one with the highest
degree of preference. degree of preference.
The Decision Process operates on routes contained in each Adj-RIB-In, The Decision Process operates on routes contained in each Adj-RIB-In,
and is responsible for: and is responsible for:
- selection of routes to be advertised to BGP speakers located in - selection of routes to be advertised to BGP speakers located in
the local speaker's autonomous system the local speaker's autonomous system
- selection of routes to be advertised to BGP speakers located in - selection of routes to be advertised to BGP speakers located in
adjacent autonomous systems neighboring autonomous systems
- route aggregation and route information reduction - route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each The Decision Process takes place in three distinct phases, each
triggered by a different event: triggered by a different event:
a) Phase 1 is responsible for calculating the degree of preference a) Phase 1 is responsible for calculating the degree of preference
for each route received from a BGP speaker located in an adjacent for each route received from a BGP speaker located in a
autonomous system, and for advertising to the other BGP speakers neighboring autonomous system, and for advertising to the other
in the local autonomous system the routes that have the highest BGP speakers in the local autonomous system the routes that have
degree of preference for each distinct destination. the highest degree of preference for each distinct destination.
b) Phase 2 is invoked on completion of phase 1. It is responsible b) Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into distinct destination, and for installing each chosen route into
the appropriate Loc-RIB. the appropriate Loc-RIB.
c) Phase 3 is invoked after the Loc-RIB has been modified. It is c) Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each responsible for disseminating routes in the Loc-RIB to each peer
adjacent BGP speaker located in an adjacent autonomous system, located in a neighboring autonomous system, according to the
according to the policies contained in the PIB. Route aggregation policies contained in the PIB. Route aggregation and information
and information reduction can optionally be performed within this reduction can optionally be performed within this phase.
phase.
9.1.1 Phase 1: Calculation of Degree of Preference 9.1.1 Phase 1: Calculation of Degree of Preference
The Phase 1 decision function shall be invoked whenever the local BGP The Phase 1 decision function shall be invoked whenever the local BGP
speaker receives an UPDATE message from a neighboring BGP speaker speaker receives an UPDATE message from a peer located in a
located in an adjacent autonomous system that advertises a new route, neighboring autonomous system that advertises a new route, a
a replacement route, or a withdrawn route. replacement route, or a withdrawn route.
The Phase 1 decision function is a separate process which completes The Phase 1 decision function is a separate process which completes
when it has no further work to do. when it has no further work to do.
RFC DRAFT August 1992
The Phase 1 decision function shall lock an Adj-RIB-In prior to The Phase 1 decision function shall lock an Adj-RIB-In prior to
operating on any route contained within it, and shall unlock it after operating on any route contained within it, and shall unlock it after
operating on all new or unfeasible routes contained within it. operating on all new or unfeasible routes contained within it.
For each newly received or replacement feasible route, the local BGP For each newly received or replacement feasible route, the local BGP
speaker shall compute a degree of preference. If the route is learned speaker shall determine a degree of preference. If the route is
from a BGP speaker in the local autonomous system, the LOCAL_PREF learned from a BGP speaker in the local autonomous system, either the
value, if present, is taken as the degree of preference. If the value of the LOCAL_PREF attribute shall be taken as the degree of
route is learned from a BGP speaker in an adjacent autonomous system preference, or the local system shall compute the degree of
or if LOCAL_PREF is not present, then the degree of preference should preference of the route based on preconfigured policy information. If
be computed based on preconfigured policy information. The exact the route is learned from a BGP speaker in a neighboring autonomous
nature of this policy information and the computation involved is a system, then the degree of preference shall be computed based on
local matter. The local speaker shall then run the internal update preconfigured policy information. The exact nature of this policy
process of 9.2.1 to select and advertise the most preferable route. information and the computation involved is a local matter. The
local speaker shall then run the internal update process of 9.2.1 to
select and advertise the most preferable route.
9.1.2 Phase 2: Route Selection 9.1.2 Phase 2: Route Selection
The Phase 2 decision function shall be invoked on completion of Phase The Phase 2 decision function shall be invoked on completion of Phase
1. The Phase 2 function is a separate process which completes when 1. The Phase 2 function is a separate process which completes when
it has no further work to do. The Phase 2 process shall consider all it has no further work to do. The Phase 2 process shall consider all
routes that are present in the Adj-RIBs-In, including those received routes that are present in the Adj-RIBs-In, including those received
from BGP speakers located in its own autonomous system and those from BGP speakers located in its own autonomous system and those
received from BGP speakers located in adjacent autonomous systems. received from BGP speakers located in neighboring autonomous systems.
The Phase 2 decision function shall be blocked from running while the The Phase 2 decision function shall be blocked from running while the
Phase 3 decision function is in process. The Phase 2 function shall Phase 3 decision function is in process. The Phase 2 function shall
lock all Adj-RIBs-In prior to commencing its function, and shall lock all Adj-RIBs-In prior to commencing its function, and shall
unlock them on completion. unlock them on completion.
If the NEXT_HOP attribute of a BGP route depicts an address to which
the local BGP speaker doesn't have a route in its Loc-RIB, the BGP
route SHOULD be excluded from the Phase 2 decision function.
For each set of destinations for which a feasible route exists in the For each set of destinations for which a feasible route exists in the
Adj-RIBs-In, the local BGP speaker shall identify the route that has: Adj-RIBs-In, the local BGP speaker shall identify the route that has:
a) the highest degree of preference of any route to the same set a) the highest degree of preference of any route to the same set
of destinations, or of destinations, or
b) is the only route to that destination, or b) is the only route to that destination, or
c) is selected as a result of the Phase 2 tie breaking rules c) is selected as a result of the Phase 2 tie breaking rules
specified in 9.1.2.1. specified in 9.1.2.1.
An alternative procedure for selecting a route may be realized if a
BGP speaker can ascertain whether a particular route the speaker
wants to select is also present in the interior routing protocol
(IGP) of the autonomous system the speaker belongs to, and that the
BGP speaker that injected the route into the IGP has this route
RFC DRAFT August 1992
installed in its Loc-RIB. A BGP speaker may select a route, provided
that the following conditions are satisfied:
a) the NLRI of the route is present in the IGP of the autonomous
system the speaker belongs to
b) the BGP speaker that injected the NLRI into the IGP has the
route in its Loc-RIB
c) the BGP speaker that injected the NLRI into the IGP will be
used as an exit point by the IGP.
The exact procedures for verifying the above conditions are specific
to a particular IGP and are outside the scope of this document.
The local speaker shall then install that route in the Loc-RIB, The local speaker SHALL then install that route in the Loc-RIB,
replacing any route to the same destination that is currently being replacing any route to the same destination that is currently being
held in the Loc-RIB. held in the Loc-RIB. The local speaker MUST determine the immediate
next hop to the address depicted by the NEXT_HOP attribute of the
selected route by performing a lookup in the IGP and selecting one of
the possible paths in the IGP. This immediate next hop MUST be used
when installing the selected route in the Loc-RIB. If the route to
the address depicted by the NEXT_HOP attribute changes such that the
immediate next hop changes, route selection should be recalculated as
specified above.
Unfeasible routes shall be removed from the Loc-RIB, and Unfeasible routes shall be removed from the Loc-RIB, and
corresponding unfeasible routes shall then be removed from the Adj- corresponding unfeasible routes shall then be removed from the Adj-
RIBs-In. RIBs-In.
9.1.2.1 Breaking Ties (Phase 2) 9.1.2.1 Breaking Ties (Phase 2)
In its Adj-RIBs-In a BGP speaker may have several routes to the same In its Adj-RIBs-In a BGP speaker may have several routes to the same
destination that have the same degree of preference. The local destination that have the same degree of preference. The local
speaker can select only one of these routes for inclusion in the speaker can select only one of these routes for inclusion in the
associated Loc-RIB. The local speaker considers all equally associated Loc-RIB. The local speaker considers all equally
preferable routes, both those received from BGP speakers located in preferable routes, both those received from BGP speakers located in
adjacent autonomous systems, and those received from other BGP neighboring autonomous systems, and those received from other BGP
speakers located in the local speaker's autonomous system. speakers located in the local speaker's autonomous system.
Ties shall be broken according to the following rules: The following tie-breaking procedure assumes that for each candidate
route all the BGP speakers within an autonomous system can ascertain
a) If the candidate routes have identical path attributes or the cost of a path (interior distance) to the address depicted by the
differ only in the NEXT_HOP attribute, select the route that was NEXT_HOP attribute of the route. Ties shall be broken according to
advertised by the BGP speaker in an adjacent autonomous system the following algorithm:
whose BGP Identifier has the lowest value. If none of the
candidate routes were received from a BGP speaker located in an
adjacent autonomous system, select the route that was advertised
by the BGP speaker in the local autonomous system whose BGP
Identifier has the lowest value.
b) If the candidate routes differ only in their NEXT_HOP and
MULTI_EXIT_DISC attributes, and the local system is configured to
RFC DRAFT August 1992
take into account MULTI_EXIT_DISC, select the route that has the
lowest value of the MULTI_EXIT_DISC attribute.
If the local system is configured to ignore MULTI_EXIT_DISC,
select the route advertised by the BGP speaker in an adjacent
autonomous system whose BGP Identifier has the lowest value. If
none of the candidate routes were received from a BGP speaker
located in an adjacent autonomous system, select the route that
was advertised by the BGP speaker in the local autonomous system
whose BGP Identifier has the lowest value.
c) If the candidate routes differ in any path attributes other
than NEXT_HOP and MULTI_EXIT_DISC, and all of the candidate routes
were advertised by the BGP speakers within the local autonomous
system, select the route that was advertised by the BGP speaker
whose BGP identifier has the lowest value.
If the candidate routes differ in any path attributes other than a) If the local system is configured to take into account
NEXT_HOP and MULTI_EXIT_DISC, and all of the candidate routes were MULTI_EXIT_DISC, and the candidate routes differ in their
advertised by the BGP speakers in adjacent autonomous systems, MULTI_EXIT_DISC attribute, select the route that has the
select the route that was advertised by the BGP speaker whose BGP lowest value of the MULTI_EXIT_DISC attribute.
identifier has the lowest value.
If the candidate routes differ in any path attributes other than b) Otherwise, select the route that has the lowest cost
NEXT_HOP and MULTI_EXIT_DISC, and some of the candidate routes (interior distance) to the entity depicted by the NEXT_HOP
were advertised by the BGP speakers in adjacent autonomous system, attribute of the route. If there are several routes with the
while others were advertised by the BGP speakers within the local same cost, then the tie-breaking shall be broken as follows:
autonomous system, the local system shall determine the BGP
speaker within the local autonomous system whose BGP identifier
has the lowest value and is advertising a candidate route
(including itself).
If this speaker is the local system, then select the route that - if at least one of the candidate routes was advertised by
was advertised by the BGP speaker in an adjacent autonomous system the BGP speaker in a neighboring autonomous system, select
whose BGP identifier has the lowest value among all other BGP the route that was advertised by the BGP speaker in a
speakers in adjacent autonomous systems. neighboring autonomous system whose BGP Identifier has the
lowest value among all other BGP speakers in neighboring
autonomous systems;
Otherwise (if the BGP identifier of the local system is not the - otherwise, select the route that was advertised by the BGP
lowest among all BGP speakers within the local autonomous system speaker whose BGP Identifier has the lowest value.
advertising a candidate route), select the route that was
advertised by the BGP speaker within the local autonomous system
whose BGP identifier has the lowest value.
9.1.3 Phase 3: Route Dissemination 9.1.3 Phase 3: Route Dissemination
The Phase 3 decision function shall be invoked on completion of Phase The Phase 3 decision function shall be invoked on completion of Phase
RFC DRAFT August 1992
2, or when any of the following events occur: 2, or when any of the following events occur:
a) when routes in a Loc-RIB to local destinations have changed a) when routes in a Loc-RIB to local destinations have changed
b) when locally generated routes learned by means outside of BGP b) when locally generated routes learned by means outside of BGP
have changed have changed
c) when a new BGP speaker - BGP speaker connection has been c) when a new BGP speaker - BGP speaker connection has been
established established
The Phase 3 function is a separate process which completes when it The Phase 3 function is a separate process which completes when it
has no further work to do. The Phase 3 Routing Decision function has no further work to do. The Phase 3 Routing Decision function
shall be blocked from running while the Phase 2 decision function is shall be blocked from running while the Phase 2 decision function is
in process. in process.
All routes in the Loc-RIB shall be processed into a corresponding All routes in the Loc-RIB shall be processed into a corresponding
entry in the associated Adj-RIBs-Out. Route aggregation and entry in the associated Adj-RIBs-Out. Route aggregation and
information reduction techniques (see 9.2.4.1) may optionally be information reduction techniques (see 9.2.4.1) may optionally be
applied. applied.
For the benefit of future support of inter-AS multicast capabilities, For the benefit of future support of inter-AS multicast capabilities,
a BGP speaker that participates in the inter-AS multicast shall a BGP speaker that participates in inter-AS multicast routing shall
advertise a route it receives from one of its external peers and advertise a route it receives from one of its external peers and if
installs in its Loc-RIB back to the peer from which the route was it installs it in its Loc-RIB, it shall advertise it back to the peer
received. For a BGP speaker that does not participate in the inter-AS from which the route was received. For a BGP speaker that does not
multicast such an advertisement is optional. When doing such an participate in inter-AS multicast routing such an advertisement is
advertisement, the NEXT_HOP attribute should be set to the address of optional. When doing such an advertisement, the NEXT_HOP attribute
the peer. An implementation may also optimize such an advertisement should be set to the address of the peer. An implementation may also
by truncating information in the AS_PATH attribute to include only optimize such an advertisement by truncating information in the
its own AS number and that of the peer that advertised the route AS_PATH attribute to include only its own AS number and that of the
(such truncation requires the ORIGIN attribute to be set to peer that advertised the route (such truncation requires the ORIGIN
INCOMPLETE). In addition an implementation is not required to pass attribute to be set to INCOMPLETE). In addition an implementation is
optional or discretionary path attributes with such an advertisement. not required to pass optional or discretionary path attributes with
such an advertisement.
When the updating of the Adj-RIBs-Out and the Forwarding Information When the updating of the Adj-RIBs-Out and the Forwarding Information
Base (FIB) is complete, the local BGP speaker shall run the external Base (FIB) is complete, the local BGP speaker shall run the external
update process of 9.2.2. update process of 9.2.2.
9.1.4 Overlapping Routes 9.1.4 Overlapping Routes
A BGP speaker may transmit routes with overlapping Network Layer A BGP speaker may transmit routes with overlapping Network Layer
Reachability Information (NLRI) to another BGP speaker. NLRI overlap Reachability Information (NLRI) to another BGP speaker. NLRI overlap
occurs when a set of destinations are identified in non-matching occurs when a set of destinations are identified in non-matching
multiple routes. Since BGP encodes NLRI using IP prefixes, overlap multiple routes. Since BGP encodes NLRI using IP prefixes, overlap
will always exhibit subset relationships. A route describing a will always exhibit subset relationships. A route describing a
RFC DRAFT August 1992
smaller set of destinations (a longer prefix) is said to be more smaller set of destinations (a longer prefix) is said to be more
specific than a route describing a larger set of destinations (a specific than a route describing a larger set of destinations (a
shorted prefix); similarly, a route describing a larger set of shorted prefix); similarly, a route describing a larger set of
destinations (a shorter prefix) is said to be less specific than a destinations (a shorter prefix) is said to be less specific than a
route describing a smaller set of destinations (a longer prefix). route describing a smaller set of destinations (a longer prefix).
The precedence relationship effectively decomposes less specific The precedence relationship effectively decomposes less specific
routes into two parts: routes into two parts:
- a set of destinations described only by the less specific - a set of destinations described only by the less specific
route, and route, and
- a set of destinations described by the overlap of the less - a set of destinations described by the overlap of the less
specific and the more specific routes specific and the more specific routes
When overlapping routes are present in the same Adj-RIB-In, the more When overlapping routes are present in the same Adj-RIB-In, the more
specific route shall take precedence, in order from more specific to specific route shall take precedence, in order from more specific to
least specific. least specific.
The set of destinations described by the overlap represents a portion The set of destinations described by the overlap represents a portion
of the less specific route that is feasible, but is not currently in of the less specific route that is feasible, but is not currently in
use. If a more specific route is later withdrawn, the set of use. If a more specific route is later withdrawn, the set of
destinations described by the overlap will still be reachable using destinations described by the overlap will still be reachable using
the less specific route. the less specific route.
If a BGP speaker receives overlapping routes, the Decision Process If a BGP speaker receives overlapping routes, the Decision Process
shall take into account the semantics of the overlapping routes. In shall take into account the semantics of the overlapping routes. In
particular, if a BGP speaker accepts the less specific route while particular, if a BGP speaker accepts the less specific route while
rejecting the more specific route from the same neighbor, then the rejecting the more specific route from the same peer, then the
destinations represented by the overlap may not forward along the ASs destinations represented by the overlap may not forward along the ASs
listed in the AS_PATH attribute of that route. Therefore, a BGP listed in the AS_PATH attribute of that route. Therefore, a BGP
speaker has the following choices: speaker has the following choices:
a) Install both the less and the more specific routes a) Install both the less and the more specific routes
b) Install the more specific route only b) Install the more specific route only
c) Install the non-overlapping part of the less specific c) Install the non-overlapping part of the less specific
route only (that implies de-aggregation) route only (that implies de-aggregation)
d) Aggregate the two routes and install the aggregated route d) Aggregate the two routes and install the aggregated route
e) Install the less specific route only e) Install the less specific route only
f) Install neither route f) Install neither route
If a BGP speaker chooses e), then it should add ATOMIC_AGGREGATE If a BGP speaker chooses e), then it should add ATOMIC_AGGREGATE
RFC DRAFT August 1992
attribute to the route. A route that carries ATOMIC_AGGREGATE attribute to the route. A route that carries ATOMIC_AGGREGATE
attribute can not be de-aggregated. That is, the NLRI of this route attribute can not be de-aggregated. That is, the NLRI of this route
can not be made more specific. Forwarding along such a route does can not be made more specific. Forwarding along such a route does
not guarantee that IP packets will actually traverse only ASs listed not guarantee that IP packets will actually traverse only ASs listed
in the AS_PATH attribute of the route. If a BGP speaker chooses a), in the AS_PATH attribute of the route. If a BGP speaker chooses a),
it must not advertise the more general route without the more it must not advertise the more general route without the more
specific route. specific route.
9.2 Update-Send Process 9.2 Update-Send Process
The Update-Send process is responsible for advertising UPDATE The Update-Send process is responsible for advertising UPDATE
messages to adjacent BGP speakers. For example, it distributes the messages to all peers. For example, it distributes the routes chosen
routes chosen by the Decision Process to other BGP speakers which may by the Decision Process to other BGP speakers which may be located in
be located in either the same autonomous system or an adjacent either the same autonomous system or a neighboring autonomous system.
autonomous system. Rules for information exchange between BGP Rules for information exchange between BGP speakers located in
speakers located in different autonomous systems are given in 9.2.2; different autonomous systems are given in 9.2.2; rules for
rules for information exchange between BGP speakers located in the information exchange between BGP speakers located in the same
same autonomous system are given in 9.2.1. autonomous system are given in 9.2.1.
Distribution of routing information between a set of BGP speakers, Distribution of routing information between a set of BGP speakers,
all of which are located in the same autonomous system, is referred all of which are located in the same autonomous system, is referred
to as internal distribution. to as internal distribution.
9.2.1 Internal Updates 9.2.1 Internal Updates
The Internal update process is concerned with the distribution of The Internal update process is concerned with the distribution of
routing information to BGP speakers located in the local speaker's routing information to BGP speakers located in the local speaker's
autonomous system. autonomous system.
When a BGP speaker receives an UPDATE message from another BGP When a BGP speaker receives an UPDATE message from another BGP
speaker located in its own autonomous system, the receiving BGP speaker located in its own autonomous system, the receiving BGP
speaker shall not re-distribute the routing information contained in speaker shall not re-distribute the routing information contained in
that UPDATE message to other BGP speakers located in its own that UPDATE message to other BGP speakers located in its own
autonomous system. autonomous system.
When a BGP speaker receives a new route from a BGP speaker in an When a BGP speaker receives a new route from a BGP speaker in a
adjacent autonomous system, it shall advertise that route to all neighboring autonomous system, it shall advertise that route to all
other BGP speakers in its autonomous system by means of an UPDATE other BGP speakers in its autonomous system by means of an UPDATE
message if any of the following conditions occur: message if any of the following conditions occur:
1) the degree of preference assigned to the newly received route 1) the degree of preference assigned to the newly received route
by the local BGP speaker is higher than the degree of preference by the local BGP speaker is higher than the degree of preference
that the local speaker has assigned to other routes that have been that the local speaker has assigned to other routes that have been
received from BGP speakers in adjacent autonomous systems, or received from BGP speakers in neighboring autonomous systems, or
RFC DRAFT August 1992
2) there are no other routes that have been received from BGP 2) there are no other routes that have been received from BGP
speakers in adjacent autonomous systems, or speakers in neighboring autonomous systems, or
3) the newly received route is selected as a result of breaking a 3) the newly received route is selected as a result of breaking a
tie between several routes which have the highest degree of tie between several routes which have the highest degree of
preference, and the same destination. preference, and the same destination (the tie-breaking procedure
is specified in 9.2.1.1).
When a BGP speaker receives an UPDATE message with a non-empty When a BGP speaker receives an UPDATE message with a non-empty
WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all
routes whose destinations was carried in this field (as IP prefixes). routes whose destinations was carried in this field (as IP prefixes).
The speaker shall take the following additional steps: The speaker shall take the following additional steps:
1) if the corresponding feasible route had not been previously 1) if the corresponding feasible route had not been previously
advertised, then no further action is necessary advertised, then no further action is necessary
2) if the corresponding feasible route had been previously 2) if the corresponding feasible route had been previously
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i) if a new route is selected for advertisement that has the i) if a new route is selected for advertisement that has the
same Network Layer Reachability Information as the unfeasible same Network Layer Reachability Information as the unfeasible
routes, then the local BGP speaker shall advertise the routes, then the local BGP speaker shall advertise the
replacement route replacement route
ii) if a replacement route is not available for advertisement, ii) if a replacement route is not available for advertisement,
then the BGP speaker shall include the destinations of the then the BGP speaker shall include the destinations of the
unfeasible route (in form of IP prefixes) in the WITHDRAWN unfeasible route (in form of IP prefixes) in the WITHDRAWN
ROUTES field of an UPDATE message, and shall send this message ROUTES field of an UPDATE message, and shall send this message
to each neighbor BGP speaker to whom it had previously to each peer to whom it had previously advertised the
advertised the corresponding feasible route. corresponding feasible route.
All feasible routes which are advertised shall be placed in the All feasible routes which are advertised shall be placed in the
appropriate Adj-RIBs-Out, and all unfeasible routes which are appropriate Adj-RIBs-Out, and all unfeasible routes which are
advertised shall be removed from the Adj-RIBs-Out. advertised shall be removed from the Adj-RIBs-Out.
9.2.1.1 Breaking Ties (Internal Updates) 9.2.1.1 Breaking Ties (Internal Updates)
If a local BGP speaker has connections to several BGP speakers in If a local BGP speaker has connections to several BGP speakers in
adjacent autonomous systems, there will be multiple Adj-RIBs-In neighboring autonomous systems, there will be multiple Adj-RIBs-In
associated with these neighbors. These Adj-RIBs-In might contain associated with these peers. These Adj-RIBs-In might contain several
several equally preferable routes to the same destination, all of equally preferable routes to the same destination, all of which were
which were advertised by BGP speakers located in adjacent autonomous advertised by BGP speakers located in neighboring autonomous systems.
systems. The local BGP speaker shall select one of these routes The local BGP speaker shall select one of these routes according to
according to the following rules: the following rules:
a) If the candidate route differ only in their NEXT_HOP and a) If the candidate route differ only in their NEXT_HOP and
RFC DRAFT August 1992
MULTI_EXIT_DISC attributes, and the local system is configured to MULTI_EXIT_DISC attributes, and the local system is configured to
take into account MULTI_EXIT_DISC attribute, select the routes take into account MULTI_EXIT_DISC attribute, select the routes
that has the lowest value of the MULTI_EXIT_DISC attribute. that has the lowest value of the MULTI_EXIT_DISC attribute.
b) In all other cases, select the route that was advertised by the b) If the local system can ascertain the cost of a path to the
entity depicted by the NEXT_HOP attribute of the candidate route,
select the route with the lowest cost.
c) In all other cases, select the route that was advertised by the
BGP speaker whose BGP Identifier has the lowest value. BGP speaker whose BGP Identifier has the lowest value.
9.2.2 External Updates 9.2.2 External Updates
The external update process is concerned with the distribution of The external update process is concerned with the distribution of
routing information to BGP speakers located in adjacent autonomous routing information to BGP speakers located in neighboring autonomous
systems. As part of Phase 3 route selection process, the BGP speaker systems. As part of Phase 3 route selection process, the BGP speaker
has updated its Adj-RIBs-Out and its Forwarding Table. All newly has updated its Adj-RIBs-Out and its Forwarding Table. All newly
installed routes and all newly unfeasible routes for which there is installed routes and all newly unfeasible routes for which there is
no replacement route shall be advertised to BGP speakers located in no replacement route shall be advertised to BGP speakers located in
adjacent autonomous systems by means of UPDATE message. neighboring autonomous systems by means of UPDATE message.
Any routes in the Loc-RIB marked as unfeasible shall be removed. Any routes in the Loc-RIB marked as unfeasible shall be removed.
Changes to the reachable destinations within its own autonomous Changes to the reachable destinations within its own autonomous
system shall also be advertised in an UPDATE message. system shall also be advertised in an UPDATE message.
9.2.3 Controlling Routing Traffic Overhead 9.2.3 Controlling Routing Traffic Overhead
The BGP protocol constrains the amount of routing traffic (that is, The BGP protocol constrains the amount of routing traffic (that is,
UPDATE messages) in order to limit both the link bandwidth needed to UPDATE messages) in order to limit both the link bandwidth needed to
advertise UPDATE messages and the processing power needed by the advertise UPDATE messages and the processing power needed by the
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9.2.3.1 Frequency of Route Advertisement 9.2.3.1 Frequency of Route Advertisement
The parameter MinRouteAdvertisementInterval determines the minimum The parameter MinRouteAdvertisementInterval determines the minimum
amount of time that must elapse between advertisement of routes to a amount of time that must elapse between advertisement of routes to a
particular destination from a single BGP speaker. This rate limiting particular destination from a single BGP speaker. This rate limiting
procedure applies on a per-destination basis, although the value of procedure applies on a per-destination basis, although the value of
MinRouteAdvertisementInterval is set on a per BGP peer basis. MinRouteAdvertisementInterval is set on a per BGP peer basis.
Two UPDATE messages sent from a single BGP speaker that advertise Two UPDATE messages sent from a single BGP speaker that advertise
feasible routes to some common set of destinations received from BGP feasible routes to some common set of destinations received from BGP
speakers in adjacent autonomous systems must be separated by at least speakers in neighboring autonomous systems must be separated by at
MinRouteAdvertisementInterval. Clearly, this can only be achieved least MinRouteAdvertisementInterval. Clearly, this can only be
achieved precisely by keeping a separate timer for each common set of
RFC DRAFT August 1992
precisely by keeping a separate timer for each common set of
destinations. This would be unwarranted overhead. Any technique which destinations. This would be unwarranted overhead. Any technique which
ensures that the interval between two UPDATE messages sent from a ensures that the interval between two UPDATE messages sent from a
single BGP speaker that advertise feasible routes to some common set single BGP speaker that advertise feasible routes to some common set
of destinations received from BGP speakers in adjacent autonomous of destinations received from BGP speakers in neighboring autonomous
systems will be at least MinRouteAdvertisementInterval, and will also systems will be at least MinRouteAdvertisementInterval, and will also
ensure a constant upper bound on the interval is acceptable. ensure a constant upper bound on the interval is acceptable.
Since fast convergence is needed within an autonomous system, this Since fast convergence is needed within an autonomous system, this
procedure does not apply for routes receives from other BGP speakers procedure does not apply for routes receives from other BGP speakers
in the same autonomous system. To avoid long-lived black holes, the in the same autonomous system. To avoid long-lived black holes, the
procedure does not apply to the explicit withdrawal of unfeasible procedure does not apply to the explicit withdrawal of unfeasible
routes (that is, routes whose destinations (expressed as IP prefixes) routes (that is, routes whose destinations (expressed as IP prefixes)
are listed in the WITHDRAWN ROUTES field of an UPDATE message). are listed in the WITHDRAWN ROUTES field of an UPDATE message).
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9.2.3.3 Jitter 9.2.3.3 Jitter
To minimize the likelihood that the distribution of BGP messages by a To minimize the likelihood that the distribution of BGP messages by a
given BGP speaker will contain peaks, jitter should be applied to the given BGP speaker will contain peaks, jitter should be applied to the
timers associated with MinASOriginationInterval, Keepalive, and timers associated with MinASOriginationInterval, Keepalive, and
MinRouteAdvertisementInterval. A given BGP speaker shall apply the MinRouteAdvertisementInterval. A given BGP speaker shall apply the
same jitter to each of these quantities regardless of the same jitter to each of these quantities regardless of the
destinations to which the updates are being sent; that is, jitter destinations to which the updates are being sent; that is, jitter
will not be applied on a "per peer" basis. will not be applied on a "per peer" basis.
The amount of jitter to be introduced shall be determined by
multiplying the base value of the appropriate timer by a random
factor which is uniformly distributed in the range from 0.75 to 1.0.
9.2.4 Efficient Organization of Routing Information 9.2.4 Efficient Organization of Routing Information
Having selected the routing information which it will advertise, a Having selected the routing information which it will advertise, a
RFC DRAFT August 1992
BGP speaker may avail itself of several methods to organize this BGP speaker may avail itself of several methods to organize this
information in an efficient manner. information in an efficient manner.
9.2.4.1 Information Reduction 9.2.4.1 Information Reduction
Information reduction may imply a reduction in granularity of policy Information reduction may imply a reduction in granularity of policy
control - after information is collapsed, the same policies will control - after information is collapsed, the same policies will
apply to all destinations and paths in the equivalence class. apply to all destinations and paths in the equivalence class.
The Decision Process may optionally reduce the amount of information The Decision Process may optionally reduce the amount of information
skipping to change at page 46, line 4 skipping to change at page 43, line 37
use may prune potentially feasible paths, since such paths are no use may prune potentially feasible paths, since such paths are no
longer listed individually as in the form of AS_SEQUENCEs. In longer listed individually as in the form of AS_SEQUENCEs. In
practice this is not likely to be a problem, since once an IP practice this is not likely to be a problem, since once an IP
packet arrives at the edge of a group of autonomous systems, the packet arrives at the edge of a group of autonomous systems, the
BGP speaker at that point is likely to have more detailed path BGP speaker at that point is likely to have more detailed path
information and can distinguish individual paths to destinations. information and can distinguish individual paths to destinations.
9.2.4.2 Aggregating Routing Information 9.2.4.2 Aggregating Routing Information
Aggregation is the process of combining the characteristics of Aggregation is the process of combining the characteristics of
RFC DRAFT August 1992
several different routes in such a way that a single route can be several different routes in such a way that a single route can be
advertised. Aggregation can occur as part of the decision process advertised. Aggregation can occur as part of the decision process
to reduce the amount of routing information that will be placed in to reduce the amount of routing information that will be placed in
the Adj-RIBs-Out. the Adj-RIBs-Out.
Aggregation reduces the amount of information that a BGP speaker must Aggregation reduces the amount of information that a BGP speaker must
store and exchange with other BGP speakers. Routes can be aggregated store and exchange with other BGP speakers. Routes can be aggregated
by applying the following procedure separately to path attributes of by applying the following procedure separately to path attributes of
like type and to the Network Layer Reachability Information. like type and to the Network Layer Reachability Information.
skipping to change at page 46, line 41 skipping to change at page 44, line 25
other case the value of the ORIGIN attribute of the aggregated other case the value of the ORIGIN attribute of the aggregated
route is INTERNAL. route is INTERNAL.
AS_PATH attribute: If routes to be aggregated have identical AS_PATH attribute: If routes to be aggregated have identical
AS_PATH attributes, then the aggregated route has the same AS_PATH AS_PATH attributes, then the aggregated route has the same AS_PATH
attribute as each individual route. attribute as each individual route.
For the purpose of aggregating AS_PATH attributes we model each AS For the purpose of aggregating AS_PATH attributes we model each AS
within the AS_PATH attribute as a tuple <type, value>, where within the AS_PATH attribute as a tuple <type, value>, where
"type" identifies a type of the path segment the AS belongs to "type" identifies a type of the path segment the AS belongs to
(e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number. If the (e.g., AS_SEQUENCE, AS_SET), and "value" is the AS number. If the
routes to be aggregated have different AS_PATH attributes, then routes to be aggregated have different AS_PATH attributes, then
the aggregated AS_PATH attribute shall satisfy all of the the aggregated AS_PATH attribute shall satisfy all of the
following conditions: following conditions:
- all tuples of the type AS_SEQUENCE in the aggregated AS_PATH - all tuples of the type AS_SEQUENCE in the aggregated AS_PATH
shall appear in all of the AS_PATH in the initial set of routes shall appear in all of the AS_PATH in the initial set of routes
to be aggregated. to be aggregated.
- all tuples of the type AS_SET in the aggregated AS_PATH shall - all tuples of the type AS_SET in the aggregated AS_PATH shall
appear in at least one of the AS_PATH in the initial set (they appear in at least one of the AS_PATH in the initial set (they
may appear as either AS_SET or AS_SEQUENCE types). may appear as either AS_SET or AS_SEQUENCE types).
- for any tuple X of the type AS_SEQUENCE in the aggregated - for any tuple X of the type AS_SEQUENCE in the aggregated
RFC DRAFT August 1992
AS_PATH which precedes tuple Y in the aggregated AS_PATH, X AS_PATH which precedes tuple Y in the aggregated AS_PATH, X
precedes Y in each AS_PATH in the initial set which contains Y, precedes Y in each AS_PATH in the initial set which contains Y,
regardless of the type of Y. regardless of the type of Y.
- No tuple with the same value shall appear more than once in - No tuple with the same value shall appear more than once in
the aggregated AS_PATH, regardless of the tuple's type. the aggregated AS_PATH, regardless of the tuple's type.
An implementation may choose any algorithm which conforms to these An implementation may choose any algorithm which conforms to these
rules. At a minimum a conformant implementation shall be able to rules. At a minimum a conformant implementation shall be able to
perform the following algorithm that meets all of the above perform the following algorithm that meets all of the above
skipping to change at page 47, line 43 skipping to change at page 45, line 26
Appendix 6, section 6.8 presents another algorithm that satisfies Appendix 6, section 6.8 presents another algorithm that satisfies
the conditions and allows for more complex policy configurations. the conditions and allows for more complex policy configurations.
ATOMIC_AGGREGATE: If at least one of the routes to be aggregated ATOMIC_AGGREGATE: If at least one of the routes to be aggregated
has ATOMIC_AGGREGATE path attribute, then the aggregated route has ATOMIC_AGGREGATE path attribute, then the aggregated route
shall have this attribute as well. shall have this attribute as well.
AGGREGATOR: All AGGREGATOR attributes of all routes to be AGGREGATOR: All AGGREGATOR attributes of all routes to be
aggregated should be ignored. aggregated should be ignored.
9.3.6 Route Selection Criteria 9.3 Route Selection Criteria
Generally speaking, the rules for comparing routes among several Generally speaking, additional rules for comparing routes among
alternatives are outside the scope of this document. There are two several alternatives are outside the scope of this document. There
exceptions: are two exceptions:
- If the local AS appears in the AS path of the new route being - If the local AS appears in the AS path of the new route being
considered, then that new route cannot be viewed as better than considered, then that new route cannot be viewed as better than
any other route. If such a route were ever used, a routing loop any other route. If such a route were ever used, a routing loop
would result. would result.
RFC DRAFT August 1992
- In order to achieve successful distributed operation, only - In order to achieve successful distributed operation, only
routes with a likelihood of stability can be chosen. Thus, an AS routes with a likelihood of stability can be chosen. Thus, an AS
must avoid using unstable routes, and it must not make rapid must avoid using unstable routes, and it must not make rapid
spontaneous changes to its choice of route. Quantifying the terms spontaneous changes to its choice of route. Quantifying the terms
"unstable" and "rapid" in the previous sentence will require "unstable" and "rapid" in the previous sentence will require
experience, but the principle is clear. experience, but the principle is clear.
9.4 Originating BGP routes
A BGP speaker may originate BGP routes by injecting routing
information acquired by some other means (e.g., via an IGP) into BGP.
A BGP speaker that originates BGP routes shall assign the degree of
preference to these routes by passing them through the Decision
Process (see Section 9.1). These routes may also be distributed to
other BGP speakers within the local AS as part of the Internal update
process (see Section 9.2.1). The decision whether to distribute non-
BGP acquired routes within an AS via BGP or not depends on the
environment within the AS (e.g., type of IGP) and should be
controlled via configuration.
Appendix 1. BGP FSM State Transitions and Actions. Appendix 1. BGP FSM State Transitions and Actions.
This Appendix discusses the transitions between states in the BGP FSM This Appendix discusses the transitions between states in the BGP FSM
in response to BGP events. The following is the list of these states in response to BGP events. The following is the list of these states
and events. and events when the negotiated Hold Time value is non-zero.
BGP States: BGP States:
1 - Idle 1 - Idle
2 - Connect 2 - Connect
3 - Active 3 - Active
4 - OpenSent 4 - OpenSent
5 - OpenConfirm 5 - OpenConfirm
6 - Established 6 - Established
BGP Events: BGP Events:
1 - BGP Start 1 - BGP Start
2 - BGP Stop 2 - BGP Stop
3 - BGP Transport connection open 3 - BGP Transport connection open
4 - BGP Transport connection closed 4 - BGP Transport connection closed
5 - BGP Transport connection open failed 5 - BGP Transport connection open failed
6 - BGP Transport fatal error 6 - BGP Transport fatal error
7 - ConnectRetry timer expired 7 - ConnectRetry timer expired
8 - Holdtime timer expired 8 - Hold Timer expired
9 - KeepAlive timer expired 9 - KeepAlive timer expired
10 - Receive OPEN message 10 - Receive OPEN message
11 - Receive KEEPALIVE message 11 - Receive KEEPALIVE message
12 - Receive UPDATE messages 12 - Receive UPDATE messages
13 - Receive NOTIFICATION message 13 - Receive NOTIFICATION message
The following table describes the state transitions of the BGP FSM The following table describes the state transitions of the BGP FSM
and the actions triggered by these transitions. and the actions triggered by these transitions.
RFC DRAFT August 1992 Event Actions Message Sent Next State
--------------------------------------------------------------------
Event Actions Message Sent Next State Idle (1)
-------------------------------------------------------------------- 1 Initialize resources none 2
Idle (1) Start ConnectRetry timer
1 Initialize resources none 2 Initiate a transport connection
Start ConnectRetry timer others none none 1
Initiate a transport connection
others none none 1
Connect(2)
1 none none 2
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Restart ConnectRetry timer none 3
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
Active (3)
1 none none 3
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Close connection 3
Restart ConnectRetry timer
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
OpenSent(4) Connect(2)
1 none none 4 1 none none 2
4 Close transport connection none 3 3 Complete initialization OPEN 4
Restart ConnectRetry timer Clear ConnectRetry timer
6 Release resources none 1 5 Restart ConnectRetry timer none 3
10 Process OPEN is OK KEEPALIVE 5 7 Restart ConnectRetry timer none 2
Process OPEN failed NOTIFICATION 1 Initiate a transport connection
others Close transport connection NOTIFICATION 1 others Release resources none 1
Release resources
OpenConfirm (5) Active (3)
1 none none 5 1 none none 3
4 Release resources none 1 3 Complete initialization OPEN 4
6 Release resources none 1 Clear ConnectRetry timer
9 Restart KeepAlive timer KEEPALIVE 5 5 Close connection 3
11 Complete initialization none 6 Restart ConnectRetry timer
Restart Holdtime timer 7 Restart ConnectRetry timer none 2
13 Close transport connection 1 Initiate a transport connection
Release resources others Release resources none 1
others Close transport connection NOTIFICATION 1
Release resources
RFC DRAFT August 1992 OpenSent(4)
1 none none 4
4 Close transport connection none 3
Restart ConnectRetry timer
6 Release resources none 1
10 Process OPEN is OK KEEPALIVE 5
Process OPEN failed NOTIFICATION 1
others Close transport connection NOTIFICATION 1
Release resources
Established (6) OpenConfirm (5)
1 none none 6 1 none none 5
4 Release resources none 1 4 Release resources none 1
6 Release resources none 1 6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 6 9 Restart KeepAlive timer KEEPALIVE 5
11 Restart Holdtime timer KEEPALIVE 6 11 Complete initialization none 6
12 Process UPDATE is OK UPDATE 6 Restart Hold Timer
Process UPDATE failed NOTIFICATION 1 13 Close transport connection 1
13 Close transport connection 1 Release resources
Release resources others Close transport connection NOTIFICATION 1
others Close transport connection NOTIFICATION 1 Release resources
Release resources
---------------------------------------------------------------------
Established (6)
1 none none 6
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 6
11 Restart Hold Timer KEEPALIVE 6
12 Process UPDATE is OK UPDATE 6
Process UPDATE failed NOTIFICATION 1
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
---------------------------------------------------------------------
The following is a condensed version of the above state transition The following is a condensed version of the above state transition
table. table.
Events| Idle | Active | Connect | OpenSent | OpenConfirm | Estab Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab
| (1) | (2) | (3) | (4) | (5) | (6) | (1) | (2) | (3) | (4) | (5) | (6)
|-------------------------------------------------------------- |---------------------------------------------------------------
1 | 2 | 2 | 3 | 4 | 5 | 6 1 | 2 | 2 | 3 | 4 | 5 | 6
| | | | | | | | | | | |
2 | 1 | 1 | 1 | 1 | 1 | 1 2 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | | | | | | | |
3 | 1 | 4 | 4 | 1 | 1 | 1 3 | 1 | 4 | 4 | 1 | 1 | 1
| | | | | | | | | | | |
4 | 1 | 1 | 1 | 3 | 1 | 1 4 | 1 | 1 | 1 | 3 | 1 | 1
| | | | | | | | | | | |
5 | 1 | 3 | 3 | 1 | 1 | 1 5 | 1 | 3 | 3 | 1 | 1 | 1
| | | | | | | | | | | |
6 | 1 | 1 | 1 | 1 | 1 | 1 6 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | | | | | | | |
7 | 1 | 2 | 2 | 1 | 1 | 1 7 | 1 | 2 | 2 | 1 | 1 | 1
| | | | | | | | | | | |
8 | 1 | 1 | 1 | 1 | 1 | 1 8 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | | | | | | | |
9 | 1 | 1 | 1 | 1 | 5 | 6 9 | 1 | 1 | 1 | 1 | 5 | 6
| | | | | | | | | | | |
10 | 1 | 1 | 1 | 1 or 5 | 1 | 1 10 | 1 | 1 | 1 | 1 or 5 | 1 | 1
| | | | | | | | | | | |
11 | 1 | 1 | 1 | 1 | 6 | 6 11 | 1 | 1 | 1 | 1 | 6 | 6
| | | | | | | | | | | |
12 | 1 | 1 | 1 | 1 | 1 | 1 or 6 12 | 1 | 1 | 1 | 1 | 1 | 1 or 6
| | | | | |
RFC DRAFT August 1992 13 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
| | | | | |
13 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
--------------------------------------------------------------- ---------------------------------------------------------------
Appendix 2. Comparison with RFC1267 Appendix 2. Comparison with RFC 1267
BGP-4 is capable of operating in an environment where a set of BGP-4 is capable of operating in an environment where a set of
reachable destinations may be expressed via a single IP prefix. The reachable destinations may be expressed via a single IP prefix. The
concept of network classes, or subnetting is foreign to BGP-4. To concept of network classes, or subnetting is foreign to BGP-4. To
accommodate these capabilities BGP-4 changes semantics and encoding accommodate these capabilities BGP-4 changes semantics and encoding
associated with the AS_PATH attribute. New text has been added to associated with the AS_PATH attribute. New text has been added to
define semantics associated with IP prefixes. These abilities allow define semantics associated with IP prefixes. These abilities allow
BGP-4 to support the proposed supernetting scheme [9]. BGP-4 to support the proposed supernetting scheme [9].
To simplify configuration this version introduces a new attribute, To simplify configuration this version introduces a new attribute,
LOCAL_PREF, that facilitates route selection procedures. LOCAL_PREF, that facilitates route selection procedures.
The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC. The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC.
A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that
certain aggregates are not de-aggregated. Another new attribute, certain aggregates are not de-aggregated. Another new attribute,
AGGREGATOR, can be added to aggregate routes in order to advertise AGGREGATOR, can be added to aggregate routes in order to advertise
which AS caused the aggregation. which AS and which BGP speaker within that AS caused the aggregation.
To insure that Hold Timers are symmetric, the Hold Time is now
negotiated on a per-connection basis. Hold Times of zero are now
supported.
Appendix 3. Comparison with RFC 1163 Appendix 3. Comparison with RFC 1163
All of the changes listed in Appendix 2, plus the following. All of the changes listed in Appendix 2, plus the following.
To detect and recover from BGP connection collision, a new field (BGP To detect and recover from BGP connection collision, a new field (BGP
Identifier) has been added to the OPEN message. New text (Section Identifier) has been added to the OPEN message. New text (Section
6.8) has been added to specify the procedure for detecting and 6.8) has been added to specify the procedure for detecting and
recovering from collision. recovering from collision.
The new document no longer restricts the border router that is passed The new document no longer restricts the border router that is passed
in the NEXT_HOP path attribute to be part of the same Autonomous in the NEXT_HOP path attribute to be part of the same Autonomous
System as the BGP Speaker. System as the BGP Speaker.
New document optimizes and simplifies the exchange of the information New document optimizes and simplifies the exchange of the information
about previously reachable routes. about previously reachable routes.
RFC DRAFT August 1992
Appendix 4. Comparison with RFC 1105 Appendix 4. Comparison with RFC 1105
All of the changes listed in Appendices 2 and 3, plus the following. All of the changes listed in Appendices 2 and 3, plus the following.
Minor changes to the RFC1105 Finite State Machine were necessary to Minor changes to the RFC1105 Finite State Machine were necessary to
accommodate the TCP user interface provided by 4.3 BSD. accommodate the TCP user interface provided by 4.3 BSD.
The notion of Up/Down/Horizontal relations present in RFC1105 has The notion of Up/Down/Horizontal relations present in RFC1105 has
been removed from the protocol. been removed from the protocol.
skipping to change at page 53, line 4 skipping to change at page 52, line 40
Appendix 5. TCP options that may be used with BGP Appendix 5. TCP options that may be used with BGP
If a local system TCP user interface supports TCP PUSH function, then If a local system TCP user interface supports TCP PUSH function, then
each BGP message should be transmitted with PUSH flag set. Setting each BGP message should be transmitted with PUSH flag set. Setting
PUSH flag forces BGP messages to be transmitted promptly to the PUSH flag forces BGP messages to be transmitted promptly to the
receiver. receiver.
If a local system TCP user interface supports setting precedence for If a local system TCP user interface supports setting precedence for
TCP connection, then the BGP transport connection should be opened TCP connection, then the BGP transport connection should be opened
with precedence set to Internetwork Control (110) value (see also with precedence set to Internetwork Control (110) value (see also
RFC DRAFT August 1992
[6]). [6]).
Appendix 6. Implementation Recommendations Appendix 6. Implementation Recommendations
This section presents some implementation recommendations. This section presents some implementation recommendations.
6.1 Multiple Networks Per Message 6.1 Multiple Networks Per Message
The BGP protocol allows for multiple networks with the same AS path The BGP protocol allows for multiple networks with the same AS path
and next-hop gateway to be specified in one message. Making use of and next-hop gateway to be specified in one message. Making use of
this capability is highly recommended. With one network per message this capability is highly recommended. With one network per message
there is a substantial increase in overhead in the receiver. Not only there is a substantial increase in overhead in the receiver. Not only
does the system overhead increase due to the reception of multiple does the system overhead increase due to the reception of multiple
messages, but the overhead of scanning the routing table for updates messages, but the overhead of scanning the routing table for updates
to BGP peers and other routing protocols (and sending the associated to BGP peers and other routing protocols (and sending the associated
skipping to change at page 54, line 5 skipping to change at page 53, line 34
acquired or a significant network topology change occurs. One method acquired or a significant network topology change occurs. One method
of doing this is to limit the rate of updates. This will eliminate of doing this is to limit the rate of updates. This will eliminate
the redundant scanning of the routing table to provide flash updates the redundant scanning of the routing table to provide flash updates
for BGP peers and other routing protocols. A disadvantage of this for BGP peers and other routing protocols. A disadvantage of this
approach is that it increases the propagation latency of routing approach is that it increases the propagation latency of routing
information. By choosing a minimum flash update interval that is not information. By choosing a minimum flash update interval that is not
much greater than the time it takes to process the multiple messages much greater than the time it takes to process the multiple messages
this latency should be minimized. A better method would be to read this latency should be minimized. A better method would be to read
all received messages before sending updates. all received messages before sending updates.
RFC DRAFT August 1992
6.2 Processing Messages on a Stream Protocol 6.2 Processing Messages on a Stream Protocol
BGP uses TCP as a transport mechanism. Due to the stream nature of BGP uses TCP as a transport mechanism. Due to the stream nature of
TCP, all the data for received messages does not necessarily arrive TCP, all the data for received messages does not necessarily arrive
at the same time. This can make it difficult to process the data as at the same time. This can make it difficult to process the data as
messages, especially on systems such as BSD Unix where it is not messages, especially on systems such as BSD Unix where it is not
possible to determine how much data has been received but not yet possible to determine how much data has been received but not yet
processed. processed.
One method that can be used in this situation is to first try to read One method that can be used in this situation is to first try to read
skipping to change at page 54, line 35 skipping to change at page 54, line 14
received. received.
6.3 Reducing route flapping 6.3 Reducing route flapping
To avoid excessive route flapping a BGP speaker which needs to To avoid excessive route flapping a BGP speaker which needs to
withdraw a destination and send an update about a more specific or withdraw a destination and send an update about a more specific or
less specific route shall combine them into the same UPDATE message. less specific route shall combine them into the same UPDATE message.
6.4 BGP Timers 6.4 BGP Timers
BGP employs five timers: ConnectRetry, Holdtime, KeepAlive, BGP employs five timers: ConnectRetry, Hold Time, KeepAlive,
MinRouteOriginationInterval, and MinRouteAdvertisementInterval MinASOriginationInterval, and MinRouteAdvertisementInterval The
Suggested value for the ConnectRetry timer is 120 seconds. Suggested suggested value for the ConnectRetry timer is 120 seconds. The
value for the Holdtime timer is 90 seconds. Suggested value for the suggested value for the Hold Time is 90 seconds. The suggested value
KeepAlive timer is 30 seconds. Suggested value for the for the KeepAlive timer is 30 seconds. The suggested value for the
MinRouteOriginationInterval is 15 minutes. Suggested value for the MinASOriginationInterval is 15 seconds. The suggested value for the
MinRouteAdvertisementInterval is 30 seconds. MinRouteAdvertisementInterval is 30 seconds.
An implementation of BGP shall allow any of these timers to be An implementation of BGP MUST allow these timers to be configurable.
configurable.
RFC DRAFT August 1992
6.5 Path attribute ordering 6.5 Path attribute ordering
Implementations which combine update messages as described above in Implementations which combine update messages as described above in
6.1 may prefer to see all path attributes presented in a known order. 6.1 may prefer to see all path attributes presented in a known order.
This permits them to quickly identify sets of attributes from This permits them to quickly identify sets of attributes from
different update messages which are semantically identical. To different update messages which are semantically identical. To
facilitate this, it is a useful optimization to order the path facilitate this, it is a useful optimization to order the path
attributes according to type code. This optimization is entirely attributes according to type code. This optimization is entirely
optional. optional.
skipping to change at page 55, line 28 skipping to change at page 54, line 45
Another useful optimization that can be done to simplify this Another useful optimization that can be done to simplify this
situation is to sort the AS numbers found in an AS_SET. This situation is to sort the AS numbers found in an AS_SET. This
optimization is entirely optional. optimization is entirely optional.
6.7 Control over version negotiation 6.7 Control over version negotiation
Since BGP-4 is capable of carrying aggregated routes which cannot be Since BGP-4 is capable of carrying aggregated routes which cannot be
properly represented in BGP-3, an implementation which supports BGP-4 properly represented in BGP-3, an implementation which supports BGP-4
and another BGP version should provide the capability to only speak and another BGP version should provide the capability to only speak
BGP-4 on a per-neighbor basis. BGP-4 on a per-peer basis.
6.8 Complex AS_PATH aggregation 6.8 Complex AS_PATH aggregation
An implementation which chooses to provide a path aggregation An implementation which chooses to provide a path aggregation
algorithm which retains significant amounts of path information may algorithm which retains significant amounts of path information may
wish to use the following procedure: wish to use the following procedure:
For the purpose of aggregating AS_PATH attributes of two routes, For the purpose of aggregating AS_PATH attributes of two routes,
we model each AS as a tuple <type, value>, where "type" identifies we model each AS as a tuple <type, value>, where "type" identifies
a type of the path segment the AS belongs to (e.g. AS_SEQUENCE, a type of the path segment the AS belongs to (e.g., AS_SEQUENCE,
AS_SET), and "value" is the AS number. Two ASs are said to be the AS_SET), and "value" is the AS number. Two ASs are said to be the
same if their corresponding <type, value> tuples are the same. same if their corresponding <type, value> tuples are the same.
The algorithm to aggregate two AS_PATH attributes works as The algorithm to aggregate two AS_PATH attributes works as
follows: follows:
a) Identify the same ASs (as defined above) within each AS_PATH a) Identify the same ASs (as defined above) within each AS_PATH
attribute that are in the same relative order within both attribute that are in the same relative order within both
AS_PATH attributes. Two ASs, X and Y, are said to be in the AS_PATH attributes. Two ASs, X and Y, are said to be in the
RFC DRAFT August 1992
same order if either: same order if either:
- X precedes Y in both AS_PATH attributes, or - Y precedes X - X precedes Y in both AS_PATH attributes, or - Y precedes X
in both AS_PATH attributes. in both AS_PATH attributes.
b) The aggregated AS_PATH attribute consists of ASs identified b) The aggregated AS_PATH attribute consists of ASs identified
in (a) in exactly the same order as they appear in the AS_PATH in (a) in exactly the same order as they appear in the AS_PATH
attributes to be aggregated. If two consecutive ASs identified attributes to be aggregated. If two consecutive ASs identified
in (a) do not immediately follow each other in both of the in (a) do not immediately follow each other in both of the
AS_PATH attributes to be aggregated, then the intervening ASs AS_PATH attributes to be aggregated, then the intervening ASs
(ASs that are between the two consecutive ASs that are the (ASs that are between the two consecutive ASs that are the
skipping to change at page 56, line 34 skipping to change at page 55, line 44
path segment; this segment is then placed in between the two path segment; this segment is then placed in between the two
consecutive ASs identified in (a) of the aggregated attribute. consecutive ASs identified in (a) of the aggregated attribute.
If as a result of the above procedure a given AS number appears If as a result of the above procedure a given AS number appears
more than once within the aggregated AS_PATH attribute, all, but more than once within the aggregated AS_PATH attribute, all, but
the last instance (rightmost occurrence) of that AS number should the last instance (rightmost occurrence) of that AS number should
be removed from the aggregated AS_PATH attribute. be removed from the aggregated AS_PATH attribute.
References References
[1] Mills, D., "Exterior Gateway Protocol Formal Specification", RFC [1] Mills, D., "Exterior Gateway Protocol Formal Specification", STD
904, BBN, April 1984. 18, RFC 904, BBN, April 1984.
[2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET [2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
Backbone", RFC 1092, T.J. Watson Research Center, February 1989. Backbone", RFC 1092, T.J. Watson Research Center, February 1989.
[3] Braun, H-W., "The NSFNET Routing Architecture", RFC 1093, [3] Braun, H-W., "The NSFNET Routing Architecture", RFC 1093,
MERIT/NSFNET Project, February 1989. MERIT/NSFNET Project, February 1989.
[4] Postel, J., "Transmission Control Protocol - DARPA Internet [4] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, DARPA, September 1981. Program Protocol Specification", RFC 793, DARPA, September 1981.
[5] Rekhter, Y., and P. Gross, "Application of the Border Gateway [5] Rekhter, Y., and P. Gross, "Application of the Border Gateway
Protocol in the Internet", RFC 1268, T.J. Watson Research Center, IBM Protocol in the Internet", T.J. Watson Research Center, IBM
Corp., ANS, October 1991. Corp., ANS, RFC 1655, T.J. Watson Research Center, MCI, July
1994.
[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol [6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", RFC 791, DARPA, September 1981. Specification", STD 5, RFC 791, DARPA, September 1981.
RFC DRAFT August 1992
[7] "Information Processing Systems - Telecommunications and [7] "Information Processing Systems - Telecommunications and
Information Exchange between Systems - Protocol for Exchange of Information Exchange between Systems - Protocol for Exchange of
Inter-domain Routeing Information among Intermediate Systems to Inter-domain Routeing Information among Intermediate Systems to
Support Forwarding of ISO 8473 PDUs", ISO/IEC JTC 1/SC 6 N7196, March Support Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993
1992.
[8] Fuller, V., Li, T., Yu, J., and Varadhan, K., "Supernetting: an [8] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
Address Assignment and Aggregation Strategy", Internet Draft, 1992. Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC 1519, BARRNet, cisco, MERIT, OARnet, September
1993.
[9] Rekhter, Y., and T. Li, "An Architecture for IP Address
Allocation with CIDR", RFC 1518, T.J. Watson Research Center,
cisco, September 1993.
Security Considerations Security Considerations
Security issues are not discussed in this memo. Security issues are not discussed in this memo.
Editors' Addresses Editors' Addresses
Yakov Rekhter Yakov Rekhter
T.J. Watson Research Center IBM Corporation T.J. Watson Research Center IBM Corporation
P.O. Box 218 P.O. Box 218
skipping to change at page 57, line 26 skipping to change at page 56, line 40
Security Considerations Security Considerations
Security issues are not discussed in this memo. Security issues are not discussed in this memo.
Editors' Addresses Editors' Addresses
Yakov Rekhter Yakov Rekhter
T.J. Watson Research Center IBM Corporation T.J. Watson Research Center IBM Corporation
P.O. Box 218 P.O. Box 218
Yorktown Heights, NY 10598 Yorktown Heights, NY 10598
Phone: (914) 945-3896 Phone: (914) 945-3896
email: yakov@watson.ibm.com EMail: yakov@watson.ibm.com
Tony Li Tony Li
cisco Systems, Inc. cisco Systems, Inc.
1525 O'Brien Drive 1525 O'Brien Drive
Menlo Park, CA 94025 Menlo Park, CA 94025
email: tli@cisco.com
EMail: tli@cisco.com
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