Multi site BGP Routing design

Ivan Pepelnjak ip at
Sat Jun 6 07:40:20 UTC 2009

> To rephrase the OP's question, would it be BCP to acquire a 
> second ASN, and without further de-aggregating, continue 
> advertising each site's IP space to the DFZ, but from 
> dissimilar ASs as opposed to the same one?

This would definitely be the best approach. You're not introducing new IP
prefixes and you're not extending AS paths, so the net effect on the global
BGP routing is zero (OK, you might have to use the 4 byte AS number :).

Just make sure that both ISPs you connect to allow you to advertise
"transit" prefixes. If site A public link goes down, but the private link is
up, site B will advertise its own address space plus site A's address space
with an extra AS number in the AS path (and the upstream ISP might filter


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