what is NSSA area in OSPF?
NSSA stands for not-so-stubby-area. As the name suggests, its basically stub area with some additional features. We know that LSA 4 and LSA 5 are not allowed in stub area. That means an ASBR (autonomous system border router) can not exist in a stub area. What if we want to configure an area as stub area when ASBR is located in that area. The solution is NSSA area.
NSSA area has to follow all the properties of stub area, that means LSA 5 is not allowed(which will reduce the size of routing table). Now, you might be thinking how ASBR will advertise the external routes to OSPF(as external routes are advertised via LSA 5). In case of NSSA area, the ASBR generates external LSAs as type 7 LSA, not LSA 5. So, all the routers in NSSA will have external routes as LSA 7.
How the other areas see LSA 7?
Please note that external LSA’s are advertised to all the routers in OSPF domain (exception: stub area). You already know by now that the ASBR in NSSA area generates the LSA 7 for external routes. These external routes need to be advertised by the ABR of NSSA area. When the ABR does advertise, it does an additional work of translating the type 7 LSA to type 5. So, all other areas will see type 5 LSA.
Does a default route (type 3) gets originated by defult as like other stub areas(stub, totally stub, totally NSSA) ?
The answer is NO. We need to advertise a default route from the ABR or ASBR in NSSA area. It does not get automatically advertised. This gives us the flexibility of advertising the default route either from ABR or ASBR.
From ASBR: ASBR needs to have a default route for originating the a default route. This default route must be learnt from non- ospf routing protocols.
From ABR : We need to use the below command in order to advertise a default route to NSSA area. ABR will advertise the default route even if it does not have a default route in the routing table.
Command to advertise from ABR : Area <area number> nssa default-information-originate
router ospf 10
Area 1 nssa default-information-originate