OSPF uses a variety of area types to divide the network which results in smaller and therefore more efficient LSDBs due to summarization at ABRs & ASBRs, also failures are limited to a single area so OSPF does not need to run a full SPF calculation across the entire network and the reduced size of the LSDB uses less memory and therefore results in quicker SPF calculations.

The Backbone Area or Area 0

The backbone area which is also configured as area 0, which is the area number which is used in the configuration. Area 0 which must be directly connected (except for a few very rare circumstances) to every other area. Area 0 must be continuous and is used for inter area routing between the OSPF areas.

Standard Area

This is the default area type which accepts link updates, route summaries and external routes.

Stub Areas

A stub area (sometimes referred to as a stubby area), is an area which does not receive type 4 or 5 LSA’s which are external routes from ASBRs. But a stubby area will receive type 3 LSAs which are summary routes from ABRs. If there are multiple ABRs in a stub area the route will be chosen which has the shortest path to the destination.

Totally Stubby Areas

Totally Stubby Areas, is a non standard area type used in Ciscos implementation of multi area OSPF. Totally Stubby Areas do not accept type 3, 4 or 5 LSAs, the ABRs only injects a single default route into the area. If there are multiple ABRs in the Totally Stubby Area each router routes to the closest (lowest metric) ABR in its area.

Not So Stubby Area (NSSA)

A Not So Stubby Area (NSSA) is a stub area which contains an an ASBR. It operates in the same way as a stub area but originates type 7 LSAs which are external routes from a ASBR in a NSSA area.

Not So Stubby Totally Stubby Area

This area name has got to be one of the craziest names I have come across, and I thought a Not So Stubby Area was bad. Its another non standard area type used by Cisco which is a Totally Stubby Area which has an ASBR which originates type 7 LSAs.