How to Configure an IBGP Session

Internal BGP (IBGP) is the protocol used for establishing BGP sessions between routers that belong to the same AS. As mentioned in an earlier post, in order to run IBGP within an AS domain, IP connectivity is needed between the routers via an IGP or static routing.

The picture below presents a configuration example of an IBGP session between two Cisco routers in the same AS:

IBGP session and configuration example

Both routers in the example belong to the same AS (AS 10) and they have a loopback interface configured. RIP is configured as an IGP protocol advertising in this way the IP address of the loopback interfaces. Finally IBGP is configured in both routers, stating their AS, the neighbor’s IP address & AS, and finally defining the loopback IP address as the BGP router id.

An important aspect of IBGP though is that a BGP peer never re-advertises the information (routes) it receives from another peer to the rest of its neighbors. The reason for this is to avoid routing information loops within an AS. This means that in order for all BGP routers within the same AS to learn all routes advertised to the AS, a full mesh topology needs to be implemented. Therefore, each router need to have a BGP session established with all other routers in the AS. There are other ways also in order to implement IBGP in large-scale networks, but this will be covered in later posts.

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IP & VoIP networking

Posted on March 5, 2013, in IP Routing and tagged , , , . Bookmark the permalink. Leave a comment.

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