When a large OSPF area is divided into smaller areas, this is called multiarea OSPF. Multiarea OSPF is useful in larger network deployments to reduce processing and memory overhead.
For instance, any time a router receives new information about the topology, as with additions, deletions, or modifications of a link, the router must rerun the SPF algorithm, create a new SPF tree, and update the routing table. The SPF algorithm is CPU-intensive and the time it takes for calculation depends on the size of the area. Too many routers in one area make the LSDB larger and increase the load on the CPU. Therefore, arranging routers into areas effectively partitions one potentially large database into smaller and more manageable databases.
Multiarea OSPF requires a hierarchical network design. The main area is called the backbone area (area 0) and all other areas must connect to the backbone area. With hierarchical routing, routing still occurs between the areas (interarea routing); while many of the tedious routing operations, such as recalculating the database, are kept within an area.
As illustrated in Figure 1, the hierarchical-topology possibilities of multiarea OSPF have these advantages:
- Smaller routing tables - There are fewer routing table entries as network addresses can be summarized between areas. For example, R1 summarizes the routes from area 1 to area 0 and R2 summarizes the routes from area 51 to area 0. R1 and R2 also propagate a default static route to area 1 and area 51.
- Reduced link-state update overhead - Minimizes processing and memory requirements, because there are fewer routers exchanging LSAs.
- Reduced frequency of SPF calculations - Localizes impact of a topology change within an area. For instance, it minimizes routing update impact, because LSA flooding stops at the area boundary.
In Figure 2, assume a link fails between two internal routers in area 51. Only the routers in area 51 exchange LSAs and rerun the SPF algorithm for this event. R1 does not receive LSAs from area 51 and does not recalculate the SPF algorithm.