OSPF Hierarchical Design

Although OSPF was developed for large networks, its implementation requires proper design and planning; this is especially important for networks with 50 or more routers. The concept of multiple separate areas inside one domain (or AS) was implemented in OSPF to reduce the amount of routing traffic and make networks more scalable.

In OSPF, there must always be one backbone area—area 0—to which all other nonbackbone areas must be directly attached. A router is a member of an OSPF area when at least one of its interfaces operates in that area. Routers that reside on boundaries between the backbone and a nonbackbone area are called Area Border Routers (ABR) and have at least one interface in each area. The boundary between the areas is within the ABR itself.

If external routes are propagated into the OSPF AS, the router that redistributes those routes is called an Autonomous System Boundary Router (ASBR). Careful design and correct mapping of areas to the network topology are important because manual summarization of routes can only be performed on ABRs and ASBRs.

Traffic sent from one nonbackbone area to another always crosses the backbone. For example, in Figure 7-10, the Area 1 ABR must forward traffic from Area 1 destined for Area 2 into the backbone. The Area 2 ABR receives the traffic from the backbone and forwards it to the appropriate destination inside Area 2.

NOTE You might encounter different terminology for the various OSPF tables:

■ OSPF neighbor table = adjacency database

■ OSPF topology table = OSPF topology database = LSDB

■ Routing table = forwarding database

Figure 7-10 Traffic from OSPF Area 1 to Area 2 Must Go Through the Backbone, Area 0

Figure 7-10 Traffic from OSPF Area 1 to Area 2 Must Go Through the Backbone, Area 0

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