BGP Support for Four-octet AS Number Space
RFC 4893

Network Working Group                                           Q. Vohra
Request for Comments: 4893                              Juniper Networks
Category: Standards Track                                        E. Chen
                                                           Cisco Systems
                                                                May 2007

               BGP Support for Four-octet AS Number Space

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Currently the Autonomous System (AS) number is encoded as a two-octet
   entity in BGP. This document describes extensions to BGP to carry the
   Autonomous System number as a four-octet entity.

1.  Introduction

   Currently the Autonomous System number is encoded as a two-octet
   entity in BGP [BGP].  To prepare for the anticipated exhaustion of
   the two-octet AS numbers, this document describes extensions to BGP
   to carry the Autonomous System number as a four-octet entity.

   More specifically, this document defines a new BGP capability, Four-
   octet AS Number Capability, that can be used by a BGP speaker to
   indicate its support for the four-octet AS numbers.  Two new
   attributes, AS4_PATH and AS4_AGGREGATOR, are introduced that can be
   used to propagate four-octet based AS path information across BGP
   speakers that do not support the four-octet AS numbers.  This
   document also specifies mechanisms for constructing the AS path
   information from the AS_PATH attribute and the AS4_PATH attribute.

   The extensions proposed in this document allow a gradual transition
   from 2-octet AS numbers to 4-octet AS numbers.

Vohra & Chen                Standards Track                     [Page 1]
RFC 4893       BGP Support for Four-octet AS Number Space       May 2007

2.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Protocol Extensions

   For the purpose of this document we define a BGP speaker that does
   not support the new 4-octet AS number extensions as an OLD BGP
   speaker, and a BGP speaker which supports the new 4-octet AS number
   extensions as a NEW BGP speaker.

   BGP carries the Autonomous System number in the "My Autonomous
   System" field of the OPEN message, in the AS_PATH attribute of the
   UPDATE message, and in the AGGREGATOR attribute of the UPDATE
   message.  BGP also carries the Autonomous System number in the BGP
   Communities attribute.

   A NEW BGP speaker uses BGP Capability Advertisements [RFC2842] to
   advertise to its neighbors (either internal or external) that it
   supports 4-octet AS number extensions, as specified in this document.

   The Capability that is used by a BGP speaker to convey to its BGP
   peer the 4-octet Autonomous System number capability, also carries
   the 4-octet Autonomous System number of the speaker in the Capability
   Value field of the Capability Optional Parameter.  The Capability
   Length field of the Capability is set to 4.

   NEW BGP speakers carry AS path information expressed in terms of 4-
   octet Autonomous Systems numbers by using the existing AS_PATH
   attribute, except that each AS number in this attribute is encoded
   not as a 2-octet, but as a 4-octet entity.  The same applies to the
   AGGREGATOR attribute - NEW BGP speakers use the same attribute,
   except that the AS carried in this attribute is encoded as a 4-octet
   entity.

   To preserve AS path information with 4-octet AS numbers across OLD
   BGP speakers, this document defines a new AS path attribute, called
   AS4_PATH.  This is an optional transitive attribute that contains the
   AS path encoded with 4-octet AS numbers.  The AS4_PATH attribute has
   the same semantics as the AS_PATH attribute, except that it is
   optional transitive, and it carries 4-octet AS numbers.

   To prevent the possible propagation of confederation path segments
   outside of a confederation, the path segment types AS_CONFED_SEQUENCE
   and AS_CONFED_SET [RFC3065] are declared invalid for the AS4_PATH
   attribute.

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