Multiarea OSPF is implemented in a two-layer area hierarchy:

Note: A regular area can have a number of subtypes, including a standard area, stub area, totally stubby area, and not-so-stubby area (NSSA). Stub, totally stubby, and NSSAs are beyond the scope of this chapter.

OSPF enforces this rigid two-layer area hierarchy. The underlying physical connectivity of the network must map to the two-layer area structure, with all non-backbone areas attaching directly to area 0. All traffic moving from one area to another area must traverse the backbone area. This traffic is referred to as interarea traffic.

The optimal number of routers per area varies based on factors such as network stability, but Cisco recommends the following guidelines: