Internet Engineering Task Force (IETF) Q. Vohra
Request for Comments: 6793 Juniper Networks
Obsoletes: 4893 E. Chen
Updates: 4271 Cisco Systems
Category: Standards Track December 2012
BGP Support for Four-Octet Autonomous System (AS) Number Space
The Autonomous System number is encoded as a two-octet entity in the
base BGP specification. This document describes extensions to BGP to
carry the Autonomous System numbers as four-octet entities. This
document obsoletes RFC 4893 and updates RFC 4271.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
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Vohra & Chen Standards Track [Page 1]RFC 6793 BGP Support for 4-Octet AS Number Space December 20121. Introduction
In the base BGP specification [RFC4271], the Autonomous System (AS)
number is encoded as a two-octet entity. To prepare for the
anticipated exhaustion of the two-octet AS numbers, this document
describes extensions to BGP to carry the AS numbers as four-octet
More specifically, this document defines a BGP capability code,
"support for four-octet AS number capability", to be used by a BGP
speaker to indicate its support for four-octet AS numbers. Two
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 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 specified in this document allow a gradual transition
from two-octet AS numbers to four-octet AS numbers.
This document obsoletes RFC 4893 and updates RFC 4271. It includes
several clarifications and editorial changes, and it specifies the
error handling for the new attributes.
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 four-octet AS number extensions as an OLD BGP
speaker, and a BGP speaker that supports the new four-octet AS number
extensions as a NEW BGP speaker.
BGP carries the AS numbers 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
AS numbers in the BGP Communities attribute.
A NEW BGP speaker uses BGP Capabilities Advertisements [RFC5492] to
advertise to its neighbors (either internal or external) that it
supports four-octet AS number extensions, as specified in this
Vohra & Chen Standards Track [Page 2]RFC 6793 BGP Support for 4-Octet AS Number Space December 2012
The capability that is used by a BGP speaker to convey to its BGP
peer the four-octet Autonomous System number capability also carries
the AS number (encoded as a four-octet entity) of the speaker in the