RFC 1265 (rfc1265) - Page 2 of 8
BGP Protocol Analysis
Alternative Format: Original Text Document
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
protocol.
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
enforced.
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
autonomous system.
To allow graceful coexistence with EGP, BGP provides support for
carrying EGP derived exterior routes. BGP also allows to carry
BGP Working Group