3.7.b Implement and troubleshoot IBGP and EBGP

3.7.b [i] EBGP, IBGP

BGP is an exterior gateway protocol (EGP), used to perform inter-domain routing in TCP/ IP networks. A BGP router needs to establish a connection (on TCP port 179) to each of its BGP peers before BGP updates can be exchanged. The BGP session between two BGP peers is said to be an external BGP (eBGP) session if the BGP peers are in different autonomous systems (AS) . A BGP session between two BGP peers is said to be an internal BGP (iBGP) session if the BGP peers are in the same autonomous systems.

By default , the peer relationship is established using the IP address of the interface closest to the peer router. However, using the neighbor update-source command, any operational interface, including the loopback interface, can be specified to be used for establishing TCP connections. This method of peering using a loopback interface is useful since it will not bring down the BGP session when there are multiple paths between the BGP peers, which would otherwise result in tearing down the BGP session if the physical interface used for establishing the session goes down. In addition to that, it also allows the routers running BGP with multiple links between them to load balance over the available paths.

To allow the redistribution of i-BGP routes into an interior gateway protocol such as IS-IS or OSPF or EIGRP, use the bgp redistribute-internal command in router configuration mode.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3872-3874).  . Kindle Edition.

http://blog.ipspace.net/2011/08/ibgp-or-ebgp-in-enterprise-network.html