Network Working Group P. Traina
Request for Comments: 1965 cisco Systems
Category: Experimental June 1996
Autonomous System Confederations for BGP
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Border Gateway Protocol  is an inter-autonomous system routing
protocol designed for TCP/IP networks.
This document describes an extension to BGP which may be used to
create a confederation of autonomous systems which is represented as
one single autonomous system to BGP peers external to the
The intention of this extension is to aid in policy administration
and reduce the management complexity of maintaining a large
The extension this document describes is widely deployed in the
It may be useful to subdivide autonomous systems with a very large
number of BGP speakers into smaller domains for purposes of
controlling routing policy via information contained in the BGP
AS_PATH attribute. For example, one may chose to consider all BGP
speakers in a geographic region as a single entity.
In addition to improvements in routing policy control, current
techniques for deploying BGP among speakers in the same autonomous
system establish a full mesh of TCP connections among all speakers
for the purpose of exchanging exterior routing information. In
autonomous systems the number of intra-domain connections that need
to be maintained by each border router can become significant.
Subdividing a large autonomous system allows a significant reduction
in the total number of intra-domain BGP connections, as the
Traina Experimental [Page 1]RFC 1965 AS Confederations for BGP June 1996
connectivity requirements simplify to the model used for inter-domain
Unfortunately subdividing an autonomous system may increase the
complexity of policy routing based on AS_PATH information for all
members of the Internet. Additionally, this division increases the
maintenance overhead of coordinating external peering when the
internal topology of this collection of autonomous systems is
Finally, dividing a large AS may unnecessarily increase the length of
the sequence portions of the AS_PATH attribute. Several common BGP
implementations can use the number of "hops" required to reach a
given destination as part of the path selection criteria. While this
is not an optimal method of determining route preference, given the
lack of other in-band information, it provides a reasonable default
behavior which is widely used across the Internet. Therefore,
division of an autonomous system into separate systems may adversely
affect optimal routing of packets through the Internet.
However, there is usually no need to expose the internal topology of
this divided autonomous system, which means it is possible to regard
a collection of autonomous systems under a common administration as a
single entity or autonomous system when viewed from outside the
confines of the confederation of autonomous systems itself.
Terms and Definitions
A collection of autonomous systems advertised as a single AS
number to BGP speakers that are not members of the confederation.
AS Confederation Identifier
An externally visible autonomous system number that identifies the
confederation as a whole.
An autonomous system that is contained in a given AS
IDRP has the concept of a routing domain confederation. An IDRP
routing domain confederation appears to IDRP speakers external to the
confederation as a single administrative entity. This extension is
based upon that work.
Traina Experimental [Page 2]RFC 1965 AS Confederations for BGP June 1996
In IDRP, routing domain confederations may be nested within each
other or disjoint portions of still larger confederations. The
algorithm BGP defines for additions to the AS_PATH attribute imposes
an additional restriction that AS confederations must be strictly
hierarchical in nature.
AS_CONFED segment type extension
Currently, BGP specifies that the AS_PATH attribute is a well-known
mandatory attribute that is composed of a sequence of AS path