5.6 Not-So-Stubby Areas
5.6.1 NSSA overview
NSSAs are a relatively new, standards-based OSPF enhancement. To understand how to use NSSAs, consider the network shown in Figure .

RTA connects to an external RIP domain, and RTB currently serves as an ABR for Area 0. If the RIP domain is not under your administrative control, what options do you have to exchange routing information between these two domains? If you are going to use dynamic routing, you could create an OSPF standard area, as shown in Figure .

However, what if the routers that you place in Area 1 do not have the required processing power or memory to run OSPF? You have learned that you can reduce the burden on OSPF routers by configuring them to participate in a stub or totally stubby area. Figure illustrates what would happen in this case.

A stub area cannot include an ASBR because Type 5 (external) LSAs are not allowed in a stub domain. The configuration shown in Figure would fail miserably.

So, how do you dynamically exchange external routing information without creating a standard OSPF area? You could configure another routing protocol, such as RIP or IGRP, in place of creating an Area 1. This may prove to be disadvantageous because an additional routing protocol must be maintained and imported into OSPF (and the RIP domain, is not under your administrative control).

With the introduction of the NSSA, you have another, more palatable option. An NSSA acts like a stub network in the sense that it does not allow Type 5 LSAs. It can also be configured to prevent floods of Type 3 and Type 4 summary LSAs, just as a totally stubby area would. However, an NSSA does allow Type 7 LSAs, which can carry external routing information and be flooded throughout the NSSA.

Note: NSSAs are supported in Cisco IOS version 11.2 and later.