Network Sales and Services Handbook (Cisco Press Networking Technology)

BGP mandates that each external router establishes an internal BGP connection with each of the other external routers in the same AS. These external peered routers will be linked by a "fully meshed graph" of route paths. The internal interconnections are used to propagate external routing information, independent of whatever Interior Gateway Protocol (IGP) might be running, such as OSPF or EIGRP.

These internal interconnections also are used among BGP routers to reach an agreement on the best route to an external destination network. This ensures that only the router managing the best path inserts information into the AS internal routing tables through the IGP in use.

There are two types of BGP peers:

• Internal BGP peers that are connected within the same Autonomous System (with Intra AS Links).

• External BGP peers that are interconnected between two Autonomous Systems (with Inter AS Links).

Figure D-6 illustrates Internal and External BGP Peers, with respective Inter- and Intra-AS Links.