In this section we will cover NSSA and Totally NSSA.

Please refer to Part-1 for Stub and totally Stubby.

Let’s look at the topology.

 

As we discussed in the previous section how stub and totally stubby areas work and we saw the routing tables for R4, R5 and R6 to see the change after the areas were converted to stub or totally stubby.

 

Now let’s work on converting area-4 to NSSA-

 

NSSA – No LSA Type-4,5

Before we even make any configuration changes, let’s look what NSSA does, it allows OSPF stub area to carry external routes.
It allows the ABR to flood the area with Type-3 Summary LSAs, one of the important aspect of NSSA is that the ASBR sends Type 7 External LSAs to the ABR, which are then translated to Type 5 LSAs when sent to area 0, that’s how NSSA combats the restriction of no Type-5 LSA.

 

Let’s convert Area-4 to NSSA-
We’ll run the nssa command on R4, R7, R9

 

Router ospf 1
Area 4 nssa

 

Now let’s take a look at the OSPF database for R4 (ABR)

We see the Type-7 LSA from generated by R7 (ASBR) and then it’s being converted to Type-5 LSA by R4.

Let’s look at the Routing table of R9.

We See the routes from Area-0 and the ABR has advertised them into NSSA as summary routes, which are allowed (Type-3 LSA). We don’t see the 10.0.0.0 network which is external. We can resolve that issue with issuing the default-information-originate command on the ABR (R4).

 

Router ospf 1
Area 4 nssa default-information-originate

We can see the default-route added to the routing table of R9, which will allow us to reach the external network 10.0.0.0.

 

Totally NSSA – No LSA Type 3, 4, 5

Totally is a modified version of Totally Stubby area, it filters Type-3 LSA and redistribution is allowed.

To convert Area-4 into Totally NSSA, we will use the keywork no-summary on the R4 (ABR).

 

R4
Area 4 nssa no-summary

 

By adding the no-summary keyword on ABR, it generates a default route and we can see that on R9.

That is all for Totally NSSA.