RFC 1774 (rfc1774) - Page 2 of 10

BGP-4 Protocol Analysis

RFC 1774                BGP-4 Protocol Analysis               March 1995


   routing policy decisions at the autonomous system level may be
   enforced.

   The key features of the protocol are the notion of path attributes
   and aggregation of network layer reachability information (NLRI).

   Path attributes provide 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
   thus far, 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-4 enhances the AS-PATH attribute to include sets of autonomous
   systems as well as lists.  This extended format allows generated
   aggregate routes to carry path information from the more specific
   routes used to generate the aggregate.

   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.

   In addition to incremental updates, BGP-4 has added the concept of
   route aggregation so that information about groups of networks may
   represented as a single entity.

   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



Traina