Multi site BGP Routing design

Steve Bertrand steve at
Sat Jun 6 01:59:54 UTC 2009

Michael K. Smith wrote:
> On 6/5/09 4:42 PM, "Steve Bertrand" <steve at> wrote:
>> 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).
> If you're running Cisco with the right IOS it looks like you could use the
> 'neighbor x.x.x.x allowas-in' command to accept your own AS.  Then you would
> just have to set your local route origination so that the appropriate routes
> were withdrawn when the backnet link goes down.

I stand corrected.

I've read about this, but does anyone have operational experience with
it that they can share?

Even though we are a very small SP, I always feel that going against the
traditional grain when doing things like this may leave a trail of
undocumented, hard-to-troubleshoot issues in the future.

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?


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