I have two routers which are IBGP neighbors of each other, and EBGP neighbors with upstream providers. I need to change the ASN runnig on the boxes with the least impact to either the IBGP or EBGP processes. After pouring over the config, it seems that there are at least two options. I am looking for some advice and best practice guidance, and ultimately, choosing a method to make the change.
The basic config is: Existing AS = 36113 New AS = 25113 RTA Config <snip> router bgp 36113 neighbor 172.16.213.6 remote-as 36113 neighbor 172.16.213.6 description IBGP Peer neighbor 172.16.213.6 version 4 neighbor 4.4.4.4 remote-as 4 neighbor 4.4.4.4 description EBGP Peer neighbor 4.4.4.4 version 4 </snip> RTB Config <snip> router bgp 36113 neighbor 172.16.213.5 remote-as 36113 neighbor 172.16.213.5 description IBGP Peer neighbor 172.16.213.5 version 4 neighbor 5.5.5.5 remote-as 5 neighbor 5.5.5.5 description EBGP Peer neighbor 5.5.5.5 version 4 </snip> Option 1: My thought is, to either use the 'neighbor x.x.x.x local-as' and 'neighbor x.x.x.x remote-as' commands on the IBGP peers to use a private AS, i.e. 65213. For example: RTA neighbor 172.16.213.6 remote-as 65213 neighbor 172.16.213.6 local-as 65213 RTB neighbor 172.16.213.6 remote-as 65213 neighbor 172.16.213.6 local-as 65213 This should cause IBGP updates in either direction to append the private AS 65213 to the public 36113, making the AS Path 65213 36113. The EBGP neighbors on each router need 'remove-private-as' to suppress the 65213 in the EBGP updates: RTA neighbor 4.4.4.4 remove-private-as RTB neighbor 5.5.5.5 remove-private-as Once this is done, IBGP updates are now from the private AS 65213. At this point, as long as the EBGP peer is configured to accept updates from either the new public AS 25113 or the old public AS 36113, the routing process on RTA can be changed, and the IBGP routes exchanged will continue. If RTA is changed before RTB, then the updates in one direction would be 65213 25113 and in the other direction would be 65213 36113. Once the routing process ID has been changed from 'router bgp 36113' to 'router bgp 25113', the private AS config could either be left in place or removed. Option 2: Would be to use the same 'neighbor x.x.x.x local-as' command on EBGP neighbors, to append the new public AS to the EBGP updates. Then, the EBGP AS path would be 25113 36113, whereas the IBGP path would be 36113. This would allow time to coordinate the new AS updates with the EBGP neighbors. Once the rest of the world is seeing the prefixes coming from the new ASN 25113, the routing process ID on each router could be changed from 'router bgp 36113' to 'router bgp 25113' and the IBGP neighbors would come up with the new ASNs. After the IBGP routes have been exchanged with AS path 25113, the 'neighbor x.x.x.x local-as 25113' can be removed, and everything will be back to normal. Are there reasons why one of these might not work? Better ideas to accomplish this? Thanks... _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
