Overview

So far we have discussed how routers within an internet exchange network reachability information. In so doing, we have considered the Routing Information Protocol (RIP), and the Open Shortest Path First Protocol (OSPF). But what if we wish to join multiple internets from multiple authorities, that each run different Interior Gateway Protocols? Here we will have some form of Backbone network that will link our various internets together. But how will we pass reachability information between them?

The Exterior Gateway Protocol (EGP) is used to convey network reachability information between neighboring routers that reside within different administrative authorities. These administrative authorities, or Autonomous Systems as they are known, are then free to decide both what routing information is to be propagated, and how this should be achieved. If we were to have a simple architecture such as that shown in Figure 12-1,our task would be simple. For example, depending upon the overall diameter of the internet, either RIP, or OSPF would suffice.

Figure 12-1: A Simple Internet

However, an Autonomous System can, and normally does, contain a number of routers that form a complete, complex, internet in its own right. The problems that EGP addresses then are those that surround what information to advertise, and to whom. Consider the following scenario shown in Figure 12-2.

Figure 12-2: A Complex Internet

In this example, we have four sites (maybe university campuses or even large buildings within a corporate end user) each with its own internet. Obviously, if these internets...