I am performing an exercise whereby one ISP is merging its AS with another. In my journey of figuring out how to do this I have found multiple useful tools. From the PE to CE side of things the neighbor <ip> local-as 64512 no-prepend replace-as dual-as command is proving very useful for being able to shift a PE router to a new AS without needing to adjust the CE equipment.
However the big challenge comes when I need to move the Core and Border routers (the ones doing the peering with the ISP that is taking over) into the new AS. It is easy enough to change the first router from the old AS to the new AS - however this of course causes all of the iBGP relationships within the core network to fail and require reconfiguration. This would be an MSO (Mass Service Outage) and require a lot of configuration. Once confederations and route reflectors get involved by brain essentially explodes.
Of the options I have found, this site seems to indicate that Juniper use something called Internal BGP Alias. It describes a rather elegant method of using this to migration router reflectors over first and then the core (P) routers with minimal disruption. Unfortunately, I'm not using Junipers and the site does state that Cisco routers can accomplish the same effect by using a combination the local-as and dual-as features. I have given this a go in my GNS3 lab but to no avail.
Below is a diagram of the simulation I am using (forgive the complicated labeling - just focus on the links between the major ASes). AS 100 is taking over AS 200 (left to right). I want to move R9 in AS 200 into AS 100 first. I basically want to move from this:
Can anyone advise how I would leverage the local-as and dual-as features to achieve this? Or if there is a smarter way to do this?
Perhaps I could make AS 200 a confederation? Or just configure all the connecting core routers to be eBGP peers with R9 on the night of the first switchover? I can see this getting excessively complicated in a real world scenario.
Regardless of the approach you adopt, BGP reconfiguration would result in outage.
So, the best could be to move traffic to unaffected link, reconfigure BGP peering and bring it up.
Unless you run MP-BGP (and MPLS), the process should not be a problem.