OSPF Areas

Areas are similar to subnets in that the routes and networks contained within can be easily summarized. In other words, areas are contiguous logical segments of the network that have been grouped together. Through the use of areas within OSPF, the network is easier to manage and provides a marked reduction in routing traffic. These benefits are gained because the topology of an area is invisible to other routers outside of the area.

Areas also allow the routers contained within them to run their own link-state database and SPF algorithm. A router runs one copy of the link-state database for each area to which it is connected.

A typical scenario for many networks as they grow and more sites are added is that the benefits of OSPF begin to degrade. For example, the link-state database continues to grow in size as the size of the network and the number of routers grow. At some point, this becomes inefficient.

The flooding of LSAs from a large number of routers can also cause congestion problems. To solve these problems, begin by segmenting your AS into multiple areas. As you group routers into areas, consider limiting the number of routers per area. Each router then has a link-state database, with entries for each router in its area. This substantially increases the efficiency of your OSPF network.