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Exterior routing is a feature of very large networks such as the Internet. Consider the structure of the Internet. Your organization's internal network is connected to a local ISP. This local ISP is itself connected to a regional ISP. Finally, the regional ISP is connected to a national backbone network.

Each of these networks is an autonomous system (AS), with its own internal routing scheme. The connections between these autonomous systems are referred to as external routing. Internal routing protocols are meant to exchange routing information with routers only within the autonomous system. Exterior routing protocols exchange routing information with other autonomous systems.