Not So Stubby Area (NSSA)
Not So Stubby Area name is the most funny name in network world, I think : ) Funny and useful. With NSSA, the External LSAs (Type 5) that is not accepted by Stub Areas, accepted as Type 7.
Normally, NSSA are do not accept Type 5 LSAs. But it convert Type 5 to Type 7 and then accept. After NSSA, at the ABR, this convert process is done again and Type 7 LSAs is converted to Type 5 LSAs.
The routes that is related to Type 7 LSAs, will be like N1 and N2 routes in the routing table.The difference between these two route type is explained before.
In NSSA, Summary LSA Type 3 is accepted like Stub Area. Beside, if a default-information originate command is used in ABR, default-route is also accepted.
To configure NSSA, all the routers in the area need to be configured as NSSA. This configuration is like below:
For default-route, the below command is used:
On our main topology, we will configure the Area 2 as not so stubby area (NSSA). The not so stubby area (NSSA) configuration will be like below on Router2, Router4 and Router5:
After this configuration, the show ip route output will be like below: