anycasting behind different ASNs?

william(at) william at
Wed Dec 6 20:25:26 UTC 2006

On Wed, 6 Dec 2006, matthew zeier wrote:

> Are there any practical issues with announcing the same route behind 
> different ASNs?
> Shortly I'll have two seperate sites (EU, US) announcing their own space 
> behind their own ASNs but have a desire to anycast a particular network out 
> of both locations as well.

What is the problem you're trying to solve that you think inconsistent 
origin AS announcements of the same network would solve which announcements
(from same locations) with same AS would not solve?

> (This is just my attempted to now have to deal with GRE tunnels between sites 
> that aren't logically connected anyways and using the same ASN).

Yes, understood. But note that different originating AS does not mean
traffic you expect to see in one location would not show up in another.
The only difference maybe when you're peering with same network from these
two locations but then you can provide distinct routing policies using

BTW - My suggestion is to consider doing it with 3 ASs, i.e.
you have "AS2 AS1" as starting path in location 1 with AS2 being
what you use for peering and AS1 being prepended to the routes.
At location 2 you do it as "AS3 AS1" with AS3 being used for peering
and AS1 prepended. Then to the outside it does not look like you
have multiple unconnected locations. But that may not achieve quite
the same effect in routing table for anycasting though...

William Leibzon
Elan Networks
william at

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