BGP-4 Protocol Analysis
RFC 1774

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Network Working Group                                  P. Traina, Editor
Request for Comments: 1774                                 cisco Systems
Category: Informational                                       March 1995

                        BGP-4 Protocol Analysis

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Introduction

   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 version 4 (BGP-4). This report summarizes
   the key features of BGP, and analyzes the protocol with respect to
   scaling and performance. This is the first of two reports on the BGP
   protocol.

   BGP-4 is an inter-autonomous system routing protocol designed for
   TCP/IP internets.  Version 1 of the BGP protocol was published in RFC
   1105. Since then BGP versions 2, 3, and 4 have been developed.
   Version 2 was documented in RFC 1163. Version 3 is documented in
   RFC1267.  The changes between versions are explained in Appendix 2 of
   [1].

   Possible applications of BGP in the Internet are documented in [2].

   Please send comments to iwg@ans.net.

Key features and algorithms of the BGP-4 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
   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

Traina                                                          [Page 1]
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                                                          [Page 2]
RFC 1774                BGP-4 Protocol Analysis               March 1995