RFC 1654 (rfc1654) - Page 2 of 56

A Border Gateway Protocol 4 (BGP-4)

RFC 1654                         BGP-4                         July 1994


   their insightful comments.

   We would like to specially acknowledge numerous contributions by
   Dennis Ferguson (ANS).

2.  Introduction

   The Border Gateway Protocol (BGP) is an inter-Autonomous System
   routing protocol.  It is built on experience gained with EGP as
   defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as
   described in RFC 1092 [2] and RFC 1093 [3].

   The primary function of a BGP speaking system is to exchange network
   reachability information with other BGP systems.  This network
   reachability information includes information on the list of
   Autonomous Systems (ASs) that reachability information traverses.
   This information is sufficient to construct a graph of AS
   connectivity from which routing loops may be pruned and some policy
   decisions at the AS level may be enforced.

   BGP-4 provides a new set of mechanisms for supporting classless
   interdomain routing.  These mechanisms include support for
   advertising an IP prefix and eliminates the concept of network
   "class" within BGP.  BGP-4 also introduces mechanisms which allow
   aggregation of routes, including aggregation of AS paths.  These
   changes provide support for the proposed supernetting scheme [8, 9].

   To characterize the set of policy decisions that can be enforced
   using BGP, one must focus on the rule that a BGP speaker advertise to
   its peers (other BGP speakers which it communicates with) in
   neighboring ASs only those routes that it itself uses.  This rule
   reflects the "hop-by-hop" routing paradigm generally used throughout
   the current Internet.  Note that some policies cannot be supported by
   the "hop-by-hop" routing paradigm and thus require techniques such as
   source routing to enforce.  For example, BGP does not enable one AS
   to send traffic to a neighboring AS intending that the traffic take a
   different route from that taken by traffic originating in the
   neighboring AS.  On the other hand, BGP can support any policy
   conforming to the "hop-by-hop" routing paradigm.  Since the current
   Internet uses only the "hop-by-hop" routing paradigm and since BGP
   can support any policy that conforms to that paradigm, BGP is highly
   applicable as an inter-AS routing protocol for the current Internet.

   A more complete discussion of what policies can and cannot be
   enforced with BGP is outside the scope of this document (but refer to
   the companion document discussing BGP usage [5]).





Rekhter & Li