Migrating from one AS to another

Unanswered Question
Apr 29th, 2010

I have a client who's company was split in half. One half is keeping their IP ranges and ASN, and the other half must be reconfigured with a new

ASN and IP block. We have obtained the new ASN and block from ARIN and have peering agreements in place. The routers are still advertising the old ASN, and we need to make a cutover. What is the most efficient way to cut to the new IPs and ASN with the minimum amount of downtime?

I can post configs if needed.



I have this problem too.
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Amit Singh Thu, 04/29/2010 - 09:50

Hi Paul,

I have not done it but how about using the BGP 'local-as' feature. Have the neighbor AS peer with your new AS as true ebgp connection with ' local-as ' command and advertise the new IP ranges to the BGP neighbor.Once everything stabilizes then start taking out the older IP ranges from the router.

I will let someone else comment on that as well and its been a while and I need to brush up my things again.


Giuseppe Larosa Fri, 04/30/2010 - 04:25

Hello Paul,

I agree with Amit the neighbor x.x.x.x local-as can be the right tool for this transition.

You may need to play with command options to achieve the desired effect



in a second time you can configure each router with new AS and you can remove the local-as option.

Hope to help


smitesh kharecha Fri, 04/30/2010 - 06:20

Exactly I agree with above technique...

In fact if you want minimum downtime, then it is the only possible solution i guess.


Smitesh Kharecha

pstebner10 Fri, 04/30/2010 - 15:17

Thanks everyone. I was already looking into this but wanted to check here to make sure that I was going down the correct path. We're going to do this next Monday and I will post back with status and ratings.

Thanks again,


Giuseppe Larosa Sat, 05/01/2010 - 00:30

Hello Paul,

during migration you may need to use route-maps to decide what to advertise to a peer in addition to the use of local-as neighbor command.

this has to be done  to avoid to advertise IP address  block 1 (associated to ASN1) with a originating AS ASN2 or viceversa

sorry if this is basic stuff, just to be more clear.

Hope to help


pstebner10 Sat, 05/01/2010 - 09:01

Thanks, Giuseppe. This is not basic stuff to me, so I do appreciate it. I am more of a firewall guy who just inherited a very complex BGP


Thanks again,
edit: We decided not to go this way; we are doing a hard cut from the old AS to the new AS. I do appreciate the help though.


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