RFC 1774 (rfc1774) - Page 2 of 10
BGP-4 Protocol Analysis
Alternative Format: Original Text Document
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