Autonomous System Confederations for BGP
RFC 1965

Document Type RFC - Experimental (June 1996; No errata)
Obsoleted by RFC 3065
Author Paul Traina 
Last updated 2013-03-02
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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 [1] 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
   autonomous system.

   The extension this document describes is widely deployed in the
   Internet today.


   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

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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

   AS Confederation
      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[2] 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.

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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
   segments.  Each AS path segment is represented by a type/length/value

   In [1], the path segment type is a 1-octet long field with the two
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