BGP 4-Byte Autonomous System

Unanswered Question
Jan 26th, 2009

Hi to All,

is there a document (or on the feature navigator) where I can find which IOS release support the BGP 4-Byte Autonomous System support ?

I have this problem too.
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Giuseppe Larosa Mon, 01/26/2009 - 07:04

Hello Roberto,


However, there is no reference to base IOS to support it

and FN has no BGP entry for 4byte ASN and no entry for RFC4893

I had seen someone speaking of 120(32)SY on GSR routers in other threads

IOS XR Release 3.4.0

Four-byte autonomous system (AS) number support was added.

Carrier supporting carrier (CSC) support for BGP was added. See Cisco IOS XR Multiprotocol Label Switching Protocol Configuration Guide for information.

Hope to help


ROBERTO TACCON Tue, 01/27/2009 - 05:14

Hi Giuseppe,

may I ask you some info when the ISP use as peering internet OLD BGP speaker 2-Byte ?

If I DON'T upgrade the IOS for support the 4-Byte AS may I "use the following consideration":

Network Considerations for a Network Which Does Not Support 4-octet AS

Have you other docs/info about ?

FOLLOWING SOME "preliminar" INFO ABOUT 4-Byte ASN support vs CISCO routers:

*** CRS & XR12000


*** C12000

12.0(32)S12 - Dec 17 2008

12.0(32)SY8 - Feb 9 2009

12.0(33)S3 - Apr 15 2009

*** C7200

12.0(32)S12 - Dec 17 2008

12.0(33)S3 - Apr 15 2009

*** C7600

12.2(33)SRE - maybe autumn 2009 < without Netflow

*** ASR1000

2.3 - Feb 2009 < without Netflow

2.5 - autumn ( Sep ) 2009 support Netflow

*** Lower IOS platforms

12.4T(pi10) - 27 Mar 2009

*** Cat6500

12.2SXI - 27 Mar 2009

*** Cat3500

Not planned

Thanks again

Roberto Taccon

Giuseppe Larosa Tue, 01/27/2009 - 07:04

Hello Roberto,

my understanding is that backward compatibility has been one of the first objective of the 4byte AS initiative.

And it cannot be different BGP makes internet working and this a major change to the protocol.

A well-known dedicated AS number AS 23456 is used by a new BGP peer (4byte capable) when presenting itself to an old BGP peer as documented in the doc you have linked.

This mechanism is standard based (it is described in the RFC4893) so it should be a viable solution also in a multivendor context.

Of course this can be a problem in a multi-homed scenario you could think the routers of two different ISPs are both in AS 23456 with effects on routing.

It is a temporay fix to allow for smooth transition.

I would expect ISPs to be able to mask this to customers exposing old style PE/edge routers, in order to do not make pressure on customers for upgrade.

Hope to help



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