Internal Border Gateway Protocol (iBGP) is a term used to describe an area of BGP operation that runs within an organisation or autonomous system. Routes can be injected from the iGBP into eBGP, vise versa.
To be able to connect to another autonomous system, it is essential to configure the following:
• The start of the routing process
• The networks to be advertised
• The BGP neighbor that the routing process will be synchronizing routing tables with over a TCP session.
iBGP basic configuration slightly differs from to that of eBGP. The configuration is not to identify and form neighborship with an eBGP peer, instead, the iBGP neighbor’s ASN is listed on the neighbor… remote-as.
The neighbor-asn command lists the same ASN as the local router’s router bgp command. However, eBGP neighbor remote-as commands list a different ASN.
To simplify this, the remote ASN is the same ASN as the local router bgp ASN.We use the diagram below as an example.
The above topology and configuration shows two iBGP router peering.
Both refer to the other router’s IP address on the Serial, FastEthernet or loopback interface between the two routers, and both refer to ASN 556.
The two routers then realize the neighbor is an iBGP neighbor because the neighbor’s ASN (556) matches the local router’s ASN, as seen on the router bgp 556 command.