BGP Protocol Analysis
Network Working Group Y. Rekhter, Editor
Request for Comments: 1265 T.J. Watson Research Center, IBM Corp.
BGP Protocol Analysis
1. Status of this Memo.
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
The purpose of this report is to document how the requirements for
advancing a routing protocol to Draft Standard have been satisfied by
the Border Gateway Protocol (BGP). This report summarizes the key
feature of BGP, and analyzes the protocol with respect to scaling and
performance. This is the first of two reports on the BGP protocol.
BGP is an inter-autonomous system routing protocol designed for the
TCP/IP internets. Version 1 of the BGP protocol was published in RFC
1105. Since then BGP versions 2 and 3 have been developed. Version 2
was documented in RFC 1163. Version 3 is documented in . The
changes between versions 1, 2 and 3 are explained in Appendix 3 of
Possible applications of BGP in the Internet are documented in .
Please send comments to email@example.com.
The BGP protocol has been developed by the IWG/BGP Working Group of
the Internet Engineering Task Force. We would like to express our
deepest thanks to Guy Almes (Rice University) who was the previous
chairman of the IWG Working Group. We also like to explicitly thank
Bob Braden (ISI) and Bob Hinden (BBN) for the review of this document
as well as their constructive and valuable comments.
4. Key features and algorithms of the BGP protocol.
This section summarizes the key features and algorithms of the BGP
protocol. BGP is an inter-autonomous system routing protocol; it is
designed to be used between multiple autonomous systems. BGP assumes
that routing within an autonomous system is done by an intra-
autonomous system routing protocol. BGP does not make any assumptions
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RFC 1265 BGP Protocol Analysis October 1991
about intra-autonomous system routing protocols employed by the
various autonomous systems. Specifically, BGP does not require all
autonomous systems to run the same intra-autonomous system routing
BGP is a real inter-autonomous system routing protocol. It imposes no
constraints on the underlying Internet topology. The information
exchanged via BGP is sufficient to construct a graph of autonomous
systems connectivity from which routing loops may be pruned and some
routing policy decisions at the autonomous system level may be
The key feature of the protocol is the notion of Path Attributes.
This feature provides BGP with flexibility and expandability. Path
attributes are partitioned into well-known and optional. The
provision for optional attributes allows experimentation that may
involve a group of BGP routers without affecting the rest of the
Internet. New optional attributes can be added to the protocol in
much the same fashion as new options are added to the Telnet
protocol, for instance. One of the most important path attributes is
the AS-PATH. As reachability information traverses the Internet, this
information is augmented by the list of autonomous systems that have
been traversed thusfar, forming the AS-PATH. The AS-PATH allows
straightforward suppression of the looping of routing information. In
addition, the AS-PATH serves as a powerful and versatile mechanism
for policy-based routing.
BGP uses an algorithm that cannot be classified as either a pure
distance vector, or a pure link state. Carrying a complete AS path in
the AS-PATH attribute allows to reconstruct large portions of the
overall topology. That makes it similar to the link state algorithms.
Exchanging only the currently used routes between the peers makes it
similar to the distance vector algorithms.
To conserve bandwidth and processing power, BGP uses incremental
updates, where after the initial exchange of complete routing
information, a pair of BGP routers exchanges only changes (deltas) to
that information. Technique of incremental updates requires reliable
transport between a pair of BGP routers. To achieve this
functionality BGP uses TCP as its transport.
BGP is a self-contained protocol. That is, it specifies how routing
information is exchanged both between BGP speakers in different
autonomous systems, and between BGP speakers within a single
To allow graceful coexistence with EGP, BGP provides support for
carrying EGP derived exterior routes. BGP also allows to carry
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