Not all the routers within an AS have to be BGP peers. For example, in some large enterprise networks, ASs generally have many more non-BGP routers. These routers communicate using an interior gateway protocol (IGP) such as the following:
Figure 3 shows that the routers in AS 53 all communicate with each other using an IGP. Routing information internal to AS 53 is redistributed from the IGP into BGP at router Chicago. Router Chicago redistributes into the IGP the routing information it receives from its external BGP peer, router Atlanta. Router Atlanta has an internal BGP link within its AS, and an external BGP link to router Topeka.
Figure 3: Interior Gateway Protocols
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