OSPF Stub Area vs Totally Stubby vs Not So Stubby Area (NSSA)
What are the advantages of making an area totally stubby instead of a regular stub area? What are the disadvantages?
By making an area totally stubby, routers in the area only see intra-area routes and a default route. This can save a lot of router memory and processor time. However, as with any type of route aggregation, the loss of routing detail makes it possible for a non-optimal route to be chosen.
Why does only the ABR need to know that the area is totally stubby rather than all routers in the configured area?
The ABR is the gateway to the rest of the area and therefore is the boundary that all inter-area LSAs need to pass through. Because of this, it only needs to filter out the type 3 LSAs and let the default route through.
Not-so-stubby areas (NSSAs) are similar to regular stub areas, except that they allow routes to be redistributed from an ASBR into that area with a special LSA type, which gets converted to a normal external route at the ABR.
Where would it be useful to make an area into an NSSA?
An NSSA is useful if you want to allow an area to inject external routes into an OSPF domain while still retaining some of the stub characteristics of the area such as not accepting external routes that are originated in other areas. Note: An NSSA does not have the default route injected by the ABR