Network Working Group Q. Vohra
Request for Comments: 4893 Juniper Networks
Category: Standards Track E. Chen
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 (C) The IETF Trust (2007).
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.
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
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
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
Vohra & Chen Standards Track [Page 2]