RFC 1654 (rfc1654) - Page 2 of 56
A Border Gateway Protocol 4 (BGP-4)
Alternative Format: Original Text Document
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