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| 6.7 |  | Stub, Totally Stubby, and Not-So-Stubby Areas |
| 6.7.6 |  | 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 the administrative control, what options are there to exchange routing information between these two domains? If dynamic routing is going to be used, an OSPF standard area could be created.

However, what if the routers that are placed in Area 1 do not have the required processing power or memory to run OSPF? It has been learned that the burden on OSPF routers can be reduced 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 does external routing information dynamically exchanged without creating a standard OSPF area? Another routing protocol could be configured, such as RIP or IGRP, in place of creating an Area 1. This may prove to be a disadvantage. This is because an additional routing protocol must be maintained and imported into OSPF and because the RIP domain is not under the administrative control.

With the introduction of the NSSA, there is another, easier 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.