6. External routing

An external routing protocol can be seen as a standardized interface for exchanging information between two autonomous systems. The main information exchanged concerns network layer reachability information (NLRI). A site that sends such an announcement via its routing protocol indicates to the other autonomous system that it is committed to routing packets to this destination.

There are several types of stand-alone systems.

• Stand-alone terminal systems

  These are sites with a single connection to the Internet, usually to an ISP. Routing to the outside world is relatively straightforward: the router at the edge of the autonomous system simply announces a default route. What's more, the autonomous system can delegate route announcements to its ISP, since there is no ambiguity as to the path...