BGP same AS # but in multiple sites

[syjeon]

Hi,

I would like to get the confirmation whether this scenario valid or not. please, see the below.

site A, B, C, D is using BGP AS # 64555 and each site is peered with BGP AS # 12345. these site does not need to peer with IBGP session. each site is totally separated as physically. but, those site is advertising different subnets each other. 

This scenario assumes all of site A, B, C, D is under MPLS VPN. if so, if ISP( AS # 12345) use BGP AS override, then would it be work? the customer doesn't want to use different AS # due to their management overhead like so.

Please, kindly let me know if there are identical environment or, it works or not.

Thanks

[John Blakley · Accepted solution]

It would work. It's not the best design though. You remove BGP's loop prevention when doing it this way. It will drop an update if it sees its own AS in the path. If you there is no way possible to have a loop, you could also configure "allowas-in" on the branch routers.

One thing to keep in mind is that when the same AS is used, it's going to be difficult to determine where a route was learned from. If you only have 4 sites (and I understand that could be for ease of math), it wouldn't be hard to create 4 different ASes.

HTH,

John