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I am trying to determine if it is possible to find the IP addresses of all routers BGP announcing an IP prefix.

For example, say 123.123.123.0/24 is being announced on three networks for the purpose of anycast. Is it possible to determine all three IP addresses of the routers where 123.123.123.0/24 is being announced?

Thanks so much in advance!

Ron F
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    A router will not announce `123.123.123.123/24`. A router could announce `123.123.123.0/24` or `123.123.123.123/32`. Depending on the network topology and routing protocol used in the routers, it _may_ be possible to look in the database of the routing protocol to see it being announced by more than one router, but this is not a given. – Ron Maupin Jul 17 '16 at 04:33
  • Sorry, 123.123.123.0/24 is what I meant (I have updated my original post). I was just using that prefix as an example and got carried away with the 123. – Ron F Jul 17 '16 at 04:36

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There are a few problems with discovering this with BGP:

With iBGP, you cannot learn prefixes from another iBGP speaker, so the routes will need to be learned from an IGP.

With eBGP, you learn through with ASes you can get to a prefix, but the AS concept hides the details of an AS's internals from other ASes. You could have three different ASes through which you could send traffic to that prefix, and each would have a BGP entry with the complete AS_PATH for each.

With something like OSPF, every router in an area knows all the routes and paths to the destination networks in the area, and it is a simple matter to look in the OSPF database to see this.

Ron Maupin
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