How to configure OSPF NSSA (Not So Stubby) Area

In the first lesson I gave an introduction to the OSPF stub areas. I also covered the stub area and totally stub area. Now we get to see the OSPF NSSA (Not So Stubby) area. Here’s the topology we will use:

And here’s the configuration. We will turn OSPF Area 1 into a stub area:

R1(config)#router ospf 1
R1(config-router)#network 192.168.12.0 0.0.0.255 area 0
R1(config-router)#redistribute connected subnets

R2(config)#router ospf 1
R2(config-router)#area 1 stub
R2(config-router)#network 192.168.12.0 0.0.0.255 area 0 
R2(config-router)#network 192.168.23.0 0.0.0.255 area 1

R3(config)#router ospf 1
R3(config-router)#area 1 stub
R3(config-router)#network 192.168.23.0 0.0.0.255 area 1

The loopback0 interface of R1 is redistributed into OSPF, I did this to test the stub area since it blocks LSA type 5. This is what the routing table of R3 looks like now:

R3#show ip route ospf 
O IA 192.168.12.0/24 [110/2] via 192.168.23.2, 00:00:42, FastEthernet0/0
O*IA 0.0.0.0/0 [110/2] via 192.168.23.2, 00:00:42, FastEthernet0/0

When we look at R3 you’ll see network 192.168.12.0 /24 and a default route as inter-area (LSA Type 3). Now to demonstrate the NSSA I will create a new loopback interface on R3:

Forum Replies

Rene just a question, why is that cisco never put a default route in NSSA? why did you need to configure it manually? does is have a reason why they didn’t put it in default?

Hello Mohammad

A forwarding address in OSPF allows extra hops to be avoided in certain routing situations because it allows a router to specify another router’s IP address as the forwarding or next hop address address. So essentially, a R1 can tell R2 that the next hop router for a destination is R3.

You can find a detailed description of forwarding addresses in OSPF and its uses at this Cisco documentation.

I hope this has been helpful!

Laz

Hi Laz,
Now got the answer . Many Thanks

br//zaman

Hi Ravi,

There is a good reason for this. With a stub or totally stub area, there is only one way out of the area and that’s the ABR.

With an NSSA area, your ASBR could also advertise a default route.

Now imagine both your ABR and ASBR advertise a default route. Which of the two default routes are used then? OSPF prefers routes in this order:

* Intra area
* Inter area
* External
* NSSA external

So if the ABR would advertise a default route by default, then the default route from the ASBR would never be used. That’s why it is not enabled by default.

Hope this helps!

Rene

It’s also worth noting, if you manually generate a default route in an NSSA area then it will appear as a type 7 within the NSSA area.

Whereas the automatic default route in a Totally Stubby NSSA area will appear as a type 3 within the NSSA area.

But, when I have tested the ABR doesn’t require a default route, even without the “always” keyword, when creating default routes in any stub areas.

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