Routing Between Networks

OSPF can be used to internetwork separate networks. Such networks could be another complete OSPF network or utilize a completely different routing protocol. Internetworking an OSPF network with a different routing protocol is a complicated task and uses a technique known as route redistribution. Routing information from the non-OSPF network is summarized and redistributed into the OSPF network. The OSPF network tags all routes learned in this manner as external. For more information on route redistribution, refer to "Internetworking with Dissimilar Protocols."

Internetworking two different OSPF networks is easier, because there is no need to convert one routing protocol's route cost information into a format that the other protocol can understand! Additionally, OSPF enables the creation of autonomous systems. An autonomous system (AS) is a self-contained network. Ostensibly, an AS would feature a single network administrator or group of administrators and use a single routing protocol.

The actual definition of an AS is somewhat fluid. In truth, it almost doesn't matter. What does matter is that OSPF permits the assignment of an AS number to a network. One very large OSPF network could be segmented into two or more autonomous systems. These systems would be interconnected via a fourth type of OSPF router, Autonomous System Border Router (ASBR). The ASBR summarizes all the routing information for its AS and forwards that summary to its counterpart ASBR in the neighboring AS. In this regard, the ASBR functions much like an area border router. The difference, obviously, is that they comprise the border between separate autonomous systems rather than areas within a single autonomous system or network.

Figure 12-5 demonstrates internetworking autonomous systems using ASBRs.

Figure 12-5: Internetworked OSPF autonomous systems.

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