Smarts BPG: Does BGP currently support 4-Byte Autonomous System Numbers (ASNs) at this time?
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Smarts BPG: Does BGP currently support 4-Byte Autonomous System Numbers (ASNs) at this time?


Article ID: 323795


Updated On:





 Does Smarts BGP 4-Byte Autonomous System Numbers (ASNs) at this time?

CISCO Nexus devices now have this feature and it is documented in the following article: 

Migration Guide for Explaining 4-Byte Autonomous System

The RFC is found here in this link:


VMware Smart Assurance - SMARTS


Border Gateway Protocol (BGP) is an Internet Engineering Task Force (IETF) standard, and is the most scalable of all routing protocols. BGP is the routing protocol of the global Internet, as well as for Service Provider private networks. BGP has expanded upon its original purpose of carrying Internet reachability information, and can now carry routes for Multicast, IPv6, VPNs, and a variety of other data. Cisco supports all IETF BGP standards, as well as most or all Internet Drafts for BGP. In addition, Cisco is an active participant in the IDR working groups at the IETF and a frequent contributor of new BGP extensions.

Cisco IOS Software now supports BGP 4-Byte Autonomous System Numbers (ASNs).
During the early time of BGP development and standardization, it was assumed that availability of a 16 bit binary number to identify the Autonomous System (AS) within BGP would have been more than sufficient. The 16 bit AS number, also known as the 2-byte AS number, provides a pool of 65536 unique Autonomous System numbers. The IANA manages the available BGP Autonomous System Numbers (ASN) pool, with the assignments being carried out by the Regional Registries.
The current consumption rate of the publicly available AS numbers suggests that the entire public 2-byte ASN pool will be fully depleted by early to middle 2011. A solution to this depletion is the expansion of the existing 2-byte AS number to a 4-byte AS number, which provides a theoretical 4,294,967,296 unique AS numbers. ARIN has made the following policy changes in conjunction with the adoption of the solution.
As of January 1, 2009, all new Autonomous System Numbers (ASNs) issued will be 4-byte by default, unless otherwise requested. "Following a globally coordinated policy, ARIN and all the Regional Internet Registries began allocating four-byte ASNs by request in January 2007; January 2009 marks the transition to allocating four-byte ASNs by default ( )".
The Cisco IOS BGP "4-byte ASN" feature allows BGP to carry a Autonomous System Number (ASN) encoded as a 4-byte entity. The addition of this feature allows an operator to use an expanded 4-byte AS number granted by IANA.

Backward compatibility between the 4-byte AS number and 2-byte AS number is provided as BGP and Multiprotocol BGP are widely deployed in ISP and MPLS VPN SP environments. Specifically, advertisement via standard based BGP capability code, two new "optional transitive" attributes: AS4_AGGREGATOR and AS4_PATH, and a newly reserved AS TRANS# 23456 for interoperability between 4-byte ASN capable and non-capable BGP speakers, are introduced to smooth migration from a 2-byte to a 4-byte ASN environment.
The implementation is compliant with IETF RFC 5396 and RFC 4893 standards.


At this time current versions of Smarts Network Protocol for BGP Domains do not offer this support for  4-Byte Autonomous System Numbers (ASNs)

This enhancement has been approved and is slated for a future release of Smarts  Smarts Network Protocol for BGP Domains