Routing Types

Given the three different types of OSPF routers illustrated in Figure 12-2, it is important to note that OSPF supports two different types of routing:

• Intra-area routing

• Inter-area routing

Their names are fairly self-evident. Intra-area routing is self- contained and limited to just the routers internal to a single area. Using the sample network first illustrated in Figure 12-1, Figure 12-3 demonstrates intra-area communications in an OSPF network.

Figure 12-3: Intra-area communications in an OSPF network.

Figure 12-3: Intra-area communications in an OSPF network.

Inter-area routing requires the exchange of data between different areas. All inter-area routing must be conducted through Area 0. Nonzero area numbers are not permitted to directly communicate with each other. This hierarchical restriction ensures that OSPF networks scale gracefully without becoming confusing morasses of links and routers.

Figure 12-4 demonstrates the proper use of Area 0 to facilitate inter-area communications in an OSPF network.

Figure 12-4: Using Area 0 to facilitate inter-area communications in an OSPF network.

The preceding examples demonstrate, at a high level, how communications work within an OSPF network. However, OSPF can also be used to communicate routing information between OSPF networks rather than just areas within a single network. This use of OSPF is examined in the following section.

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