Not-So-Stubby Areas

Not-so-stubby areas (NSSAs) are defined in RFC 1587, "The OSPF NSSA Option." NSSAs can be useful for ISPs and large institutions that need to connect to a remote site that runs a different routing protocol. NSSAs allow OSPF to import external routes into a stub area. This is a direct design violation of a regular stub area, and that is why a new RFC was introduced. By defining this new type of area, a new LSA type (Type 7) was also introduced. Using the new LSA, OSPF can now handle this apparent contradiction of importing external routes into a stub area.

NSSAs are useful for instances where you have no Internet transit link and you have to redistribute a legacy RIP network into a stub area, but you still have only a single exit point to other OSPF areas. There are many applications for this because many devices do not speak OSPF (or do not speak it well); these devices can speak RIP.

Figure below illustrates a typical NSSA network topology.