We have to merge two networks (ASNs) to one network. Each network has multiple peerings and routers so we cannot just change all instances of bgp to the new ASN. We have to operate the old and the new ASN simultanously and allow our peers to peer with both ASN over the same link with the same IP address. After all peers are migrated to the new ASN we will remove the old ASN completely and give it back to the RIR.
Is there any best practise how to achieve this? Can I go with confederation? And if yes how?
Yes, confederations are used to migrate to new AS while still keeping the old AS during migration process. In terms of setup you just need to specify “confederation-as” and “confederation-peers” in BGP instance configuration.
That doesn’t seem to work as expected. When I put the peer AS in “Confederated Peers” I can ONLY establish a connection via the original AS. If I remove it from “Confederated Peers” I can ONLY establish a connection via the new AS (put in Confederation). But I want to be able to connect by BOTH AS during a transition time.
Confederation is really designed to allow eBGP style policy enforcement within an iBGP AS and break up smaller sub-ASNs within one logical ASN.
I’ve done migration and merging of Public ASNs a numer of times and what you really want to look at is running different instances of BGP on routers that need to participate in BOTH ASNs and then convert the routers in the Legacy ASN one by one.
You’ll need to work with your upstreams to make sure the routing registry is updated to announce prefixes from the legacy ASN via the new ASN.