Internetworking Nonadjacent Networks

Routing between nonadjacent networks is, simultaneously, the most complicated and useful type of routing. Two networks can use a third network as an intermediary. It is highly likely that the three different networks will use different routing protocols, routed protocols, and address architectures. Therefore, the boundary router's job is to overcome these obstacles to communication while also guarding the border of its network.

Figure 4-6 illustrates routing between nonadjacent small networks. Figure 4-6: Routing between nonadjacent networks.

The border router of each private network in this illustration needs to protect the border of its network from unwanted intrusion. Given that the two networks that need to communicate aren't adjacent and the intermediary network is beyond their control, the risks of unwanted intrusion are much higher than if they were directly internetworked. Therefore, the network administrators must develop a set of criteria for allowing specific external users into their network, while disallowing access to everyone else. The border router would implement these criteria in an ACL.

Another responsibility of the border router would be to summarize the internal routes and redistribute this information to the networks beyond. This enables users outside the bounds of the private network to access its end systems. If this routing information weren't distributed, no one outside that private network would be able to access its end systems.

Finally, it is highly likely that the border routers will have to be configured to use multiple routing protocols. An interior gateway protocol will likely be selected for intranetwork routing purposes. Calculating routes across the internetwork, however, might require a different protocol---one that features stronger support for route summarization.

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