Single-area OSPF is useful in smaller networks where the web of router links is not complex, and paths to individual destinations are easily deduced.
However, if an area becomes too big, the following issues must be addressed (see the figure for illustration):
- Large routing table - OSPF does not perform route summarization by default. If the routes are not summarized, the routing table can become very large, depending on the size of the network.
- Large link-state database (LSDB) - Because the LSDB covers the topology of the entire network, each router must maintain an entry for every network in the area, even if not every route is selected for the routing table.
- Frequent SPF algorithm calculations - In a large network, changes are inevitable, so the routers spend many CPU cycles recalculating the SPF algorithm and updating the routing table.
To make OSPF more efficient and scalable, OSPF supports hierarchical routing using areas. An OSPF area is a group of routers that share the same link-state information in their link-state databases.