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:

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Hi Laz,
Now got the answer . Many Thanks

br//zaman

Thank you for responding, I understand NSSA area doesn’t generate default route. My question here would be why only NSSA area behaves like this and doesn’t generate default route.

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.

Hi Vlad,

I think the behavior of the NSSA has changed. When I wrote this post I used IOS 12.4 (2013…long time ago). I just tried it on IOS 15:

R1#show version | include IOS
Cisco IOS Software, IOSv Software (VIOS-ADVENTERPRISEK9-M), Version 15.6(3)M2, RELEASE SOFTWARE (fc2)

Even without a default route on R2, I get the default route on R3:

R3#show ip route ospf | include 0.0.0.0/0
O*N2  0.0.0.0/0 [110/1] via 192.168.23.2, 00:11:12, GigabitEthernet0/1

I updated the lesson to get rid of the always parameter.

Rene

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