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4byte AS support on 7301 router issue.

Hi all,

We've been recently assigned our own PI space and a 4byte AS number for our organization. We now have a requirement to peer with two ISP's for redundancy etc.

However, we currently have 2 7301 routers on the outside which don;t support 4byte AS numbers, the first ISP I need to peer with also does not provide current support for 4byte AS's.

I'm fine with traditional peering, acceptance of default and customer routes, RFC1918 filtering and path manipulation, however, how do I inform the Internet about our AS when nothing in the path seems to support it, I've read briefly about ASTRANS#23456 but can't find anything about configuring such a scenario, how will the Internet see my AS???

All assistance greatly received.

Everyone's tags (3)
Cisco Employee

Re: 4byte AS support on 7301 router issue.


You don't have the choice, you need to upgrade your router to support 4B ASN. The following version available for the 7301 support it:

- 12.4(24)T or higher

- 12.2(33)SRE

Once it's done, the support of 4B asn will be negotiated with your ISP during the BGP session establishment and if it doesn't support it, your router will automatically replace your AS number by 23456.

routers supporting 4B ASN must be backward compatible with the one's which doesn't support it so everything is already taken care of and you just do your BGP configuration as usual. The advantage is you now you will any 4B ASN if present in the BGP AS path.

Here is a link for more information:



New Member

Re: 4byte AS support on 7301 router issue.


Sorry for the late reply, I upgraded the 7301 to the IOS you suggested. No problems, the 7301 now supports ASDOT AS numbers, I put my math to work and worked out the correct dotted AS and successfully peered with my ISP.

The 7301 does only support ASDOT for 4byte AS, but hey it works.




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