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:

R3(config)#interface loopback 0
R3(config-if)#ip address 3.3.3.3 255.255.255.0
R3(config-if)#exit
R3(config)#router ospf 1
R3(config-router)#redistribute connected subnets

I’ll add a new loopback interface and try to redistribute it into OSPF on R3, here’s what you’ll see:

R3# %OSPF-4-ASBR_WITHOUT_VALID_AREA: Router is currently an ASBR while having only one area which is a stub area

The stub and totally stub area block LSA Type 5 so it’s impossible to have an ASBR within these areas. Let me show you how the NSSA solves this problem.

R2(config)#router ospf 1
R2(config-router)#no area 1 stub
R2(config-router)#area 1 nssa

R3(config)#router ospf 1
R3(config-router)#no area 1 stub
R3(config-router)#area 1 nssa

We’ll convert area 1 into a NSSA area. What is different this time?

R3#show ip route ospf
O IA 192.168.12.0/24 [110/2] via 192.168.23.2, 00:00:44, FastEthernet0/0

First of all you can see that it only allows inter-area routes. LSA Type 5 is blocked but I also don’t see any default routes…you have to do this yourself for the NSSA area otherwise R3 will be unable to reach network 1.1.1.0 /24.

R2(config)#router ospf 1
R2(config-router)#area 1 nssa default-information-originate

We’ll add the default route on R2. Be careful: I’m using a different command this time…area nssa default-information-originate.

R3#show ip route ospf
O IA 192.168.12.0/24 [110/2] via 192.168.23.2, 00:06:21, FastEthernet0/0
O*N2 0.0.0.0/0 [110/1] via 192.168.23.2, 00:00:54, FastEthernet0/0

Now we see a default route on R3, it shows up as a N2 route (OSPF NSSA External Type 2).

The reason I configured the NSSA area is so we could redistribute the loopback0 interface on R3 into OSPF. Did this work? Let’s find out:

R1#show ip route ospf 
     3.0.0.0/24 is subnetted, 1 subnets
O E2    3.3.3.0 [110/20] via 192.168.12.2, 00:07:25, FastEthernet0/0
O IA 192.168.23.0/24 [110/2] via 192.168.12.2, 00:07:41, FastEthernet0/0

There you go. We can see network 3.3.3.0 /24 as an external type 2 route on R1. That’s all there is to it!

hostname R2
!
interface FastEthernet0/0
 ip address 192.168.12.2 255.255.255.0
!
interface FastEthernet1/0
 ip address 192.168.23.2 255.255.255.0
!
router ospf 1
 area 1 nssa default-information-originate
 network 192.168.12.0 0.0.0.255 area 0
 network 192.168.23.0 0.0.0.255 area 1
!
end

hostname R1
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.0
!
interface FastEthernet0/0
 ip address 192.168.12.1 255.255.255.0
!
router ospf 1
 redistribute connected subnets
 network 192.168.12.0 0.0.0.255 area 0
!
end

hostname R3
!
interface Loopback0
 ip address 3.3.3.3 255.255.255.0
!
interface FastEthernet0/0
 ip address 192.168.23.3 255.255.255.0
!
router ospf 1
 area 1 nssa
 redistribute connected subnets
 network 192.168.23.0 0.0.0.255 area 1
!
end