How to configure OSPF Totally NSSA Area

OSPF has different special area types. I covered these before:

• Stub area
• Totally Stub area
• NSSA

Now it’s time to demonstrate the totally NSSA area. Here is 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

Let’s see what the routing table of R3 looks like:

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). Because area 1 is a stub area, we won’t see 1.1.1.0/24 as external type 2 (LSA type 5). First, we’ll create a loopback interface on R3 and redistribute it into OSPF to show you the NSSA area.

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. First, I’ll change area 1 into an NSSA to allow this ASBR: