Multi site BGP Routing design

Steve Bertrand steve at
Fri Jun 5 23:42:43 UTC 2009

Justin Krejci wrote:

> If the private link between the two sites fails, will BGP allow for us to
> access the IP subnets at site 2 from site 1 via the internet given that both
> sites are advertising under the same ASN?

No, because your router at site 2 will not accept any prefix with its
own AS in the AS_PATH (which site 1 would be advertising from).

> Is this a case where having multiple ASNs makes sense to treat each site as
> remote peers to each other?

Unless someone else has any better advice (I'm sure they do), you will
need two separate public ASNs. Site 1 advertises it's space out of AS1,
and site 2 advertises it's space from AS2.

If you do that, it may be best if you have an eBGP session between the
two PoPs using med/pref to ensure the direct link is preferred if it is
up. (I've never had to do iBGP between two sites like this before, but I
do know that eBGP is preferred over iBGP).

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