Stubby areas further optimizes the processing of routes by removing all external routes from being advertised into its area, taking advantage of the fact that certain areas don’t need to know all the specific external routes. Nonbackbone areas that do not contain ASBRs would only learn of external routes via ABRs, so this can easily be improved by simply replacing all those external routes with a single default route.
This type of area is what’s referred to as a stubby area, simply an area that does not mediate an external connection. The only scenario where it would be necessary to learn all external routes would be if you had multiple ASBRs and multiple ABRs connecting to this area to allow the nonbackbone area to determine the true least cost path in that scenario.
Specifically if an area is deemed stub, ABRs will stop sending type 4 and type 5 LSA into the area. Also routers in this area will no longer accept type 4 or 5 LSAs, a default route will be injected into the area by the ABR as a type 3 LSA, preserving the visibility of inter-area and intra-area routing.
To configure stubby areas you would use the following commands based on the type of Stubby area.
NSSA’s or not so stubby areas allow the creation of type 7 lsas from ASBR external routes that exist in the area, these type 7 external routes are allowed to be advertised into other areas, hence why this area type is referred to as not so stubby.
Totally stubby areas do not allow type 7 routes from being advertised to other areas.