What is NSSA?

An NSSA (Not-So-Stubby Area) is an OSPF area that blocks external routes from the rest of the network, like a stub area, but may still contain its own ASBR.

A plain stub area keeps its routers' databases small by refusing Type 5 (AS-External) LSAs and relying on a default route instead. The catch is that no router inside a stub area can redistribute routes into OSPF, because that would need Type 5 LSAs. NSSA, defined in RFC 3101, removes that restriction. An ASBR inside the NSSA describes its external routes in Type 7 (NSSA-External) LSAs, which are flooded only within the NSSA. The ABR connecting the NSSA to area 0 translates them into ordinary Type 5 LSAs for the rest of the network; if there are several ABRs, one of them is chosen as the translator. External routes from elsewhere still stay out. A totally NSSA goes further and also blocks Type 3 summary LSAs, so its routers see only their own area plus a default route.

The typical use is a branch or remote site that reaches everything through the core but has one local connection to inject, perhaps a static route to a partner network or a small connected segment. One thing to watch is the default route: depending on the implementation and the NSSA flavour, the ABR may not inject one into a plain NSSA automatically, so routers inside can end up with no way out unless you configure it. Every router in the area must agree that it is an NSSA, or their adjacencies won't form, since the area type is checked in hello packets. When reading the routing table, Type 7 routes appear as NSSA externals (often N1/N2), separate from E1/E2. The OSPF Fundamentals lesson covers NSSA alongside the other area types and every LSA type.