RFC 1265 (rfc1265) - Page 2 of 8

BGP Protocol Analysis

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