How to configure OSPF Stub Area

Previously I explained the different OSPF special area types called “stub areas”. In the next series of lessons I will show you the configuration of each OSPF stub type. Let’s start with the “normal” stub type. This is the topology that we will use:

In the picture above we have 2 areas…area 0 and area 1. I’ll use this topology to demonstrate all the OSPF area types to you. Let me show you the configuration:

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)#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)#network 192.168.23.0 0.0.0.255 area 1

I advertised all the interfaces in the correct OSPF areas with the exception of the loopback0 interface on R1. This interface is redistributed into OSPF so it becomes a LSA Type 5.

R3#show ip route ospf 
O IA 192.168.12.0/24 [110/2] via 192.168.23.2, 00:08:53, FastEthernet0/0
     1.0.0.0/24 is subnetted, 1 subnets
O E2    1.1.1.0 [110/20] via 192.168.23.2, 00:01:16, FastEthernet0/0

When we look at R3 you’ll see network 192.168.12.0 /24 as inter-area (LSA Type 3) and 1.1.1.0 /24 as external type 2 (LSA Type 5). Now let’s change area 1 in a stub area:

R2(config)#router ospf 1
R2(config-router)#area 1 stub

R3(config)#router ospf 1
R3(config-router)#area 1 stub

This is how you change it into a stub area. Now let’s see what has changed:

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

The stub area blocks LSA type 5 so you don’t see network 1.1.1.0 /24 anymore. It does however insert a default route. 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 stub
 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
 duplex auto
 speed auto
 media-type rj45
!
router ospf 1
 redistribute connected subnets
 network 192.168.12.0 0.0.0.255 area 0
!
end

hostname R3
!
interface FastEthernet0/0
 ip address 192.168.23.3 255.255.255.0
!
router ospf 1
 area 1 stub
 network 192.168.23.0 0.0.0.255 area 1
!
end

In the next lesson I will cover the totally stub area. Feel free to ask any questions!