Let us consider the scenario where two ASN's are announcing the same IP space however one has a summary route.

ASN 123:

ASN 234:

Will traffic prefer to go to ASN 234, as it is the most specific route? Also, would anything stop either ASN from using a redistributed static route to handle the traffic after it is inside of their ASN?

  • Those are two completely different prefixes, so they are not announcing the same space, and both will be installed in the routing table. It just so happens that one is a subnet of the other.
    – Ron Maupin
    Commented Jul 17, 2018 at 20:45

3 Answers 3


The basis of routing is that the more specific match is the primary criteria. So the /23 prefix will be preferred.

eBGP is used to exchange routes between AS. How routing is performed inside an AS is up to the owner of the AS, he can do whatever he wants, even static routing (or even iBGP).


More specific match is always going to be selected, so all the traffic will go to 234. You can't control how other ASNs are going to do their routing. If they decide to announce more specific static via redistribution then all the traffic will use it. However in normal situation ISP is not going to use static routes for subnet which is not part of their address space.


BGP uses TCP as the transport protocol, on port 179. Two BGP routers form a TCP connection between one another. These routers are peer routers. The peer routers exchange messages to open and confirm the connection parameters. BGP routers exchange network reachability information. This information is mainly an indication of the full paths that a route must take in order to reach the destination network. The paths are BGP AS numbers. This information helps in the construction of a graph of ASs that are loop-free. The graph also shows where to apply routing policies in order to enforce some restrictions on the routing behavior. Any two routers that form a TCP connection in order to exchange BGP routing information are "peers" or "neighbors". BGP peers initially exchange the full BGP routing tables. After this exchange, the peers send incremental updates as the routing table changes. BGP keeps a version number of the BGP table. The version number is the same for all the BGP peers. The version number changes whenever BGP updates the table with routing information changes. The send of keepalive packets ensures that the connection between the BGP peers is alive. Notification packets go out in response to errors or special conditions.

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