OSPF - Virtual Links

When you are using OSPF as your routing protocol there is one thing that you must consider before the actual network implementation. An OSPF network must contain an area 0 and might contain different areas. All OSPF areas must be connected to area 0, what we called backbone area. Usually, when our network expends, we might end up having non-zero area connected to a non-zero area and this of course won't allow communication between the areas. The solution to this case is to use a virtual link to bridge across an intermediate area. Let's see an example.

Topology

R2(config)# router ospf 1

R2(config)# router-id 2.2.2.2

R2(config)# network 10.23.23.0 0.0.0.3 area 23

R3(config)# router ospf 1

R3(config)# router-id 3.3.3.3

R3(config)# network 10.23.23.0 0.0.0.3 area 23

R3(config)# network 10.34.34.0 0.0.0.3 area 34

R4(config)# router ospf 1

R4(config)# router-id 4.4.4.4

R4(config)# network 10.34.34.0 0.0.0.3 area 34

R4(config)# network 10.46.46.0 0.0.0.3 area 0

R6(config)# router ospf 1

R6(config)# router-id 6.6.6.6

R6(config)# network 10.46.46.0 0.0.0.3 area 0

As you can see the area 23 is not connected to area 0 and R4 won't have the route 10.23.23.0 in its routing table. To solve this issue we have to configure a virtual-link between R3 and R4 (transit area).

R3(config)# router ospf 1

R3(config)# area 34 virtual-link 4.4.4.4

R4(config)# router ospf 1

R4(config)# area 34 virtual-link 3.3.3.3

After the virtual-link configuration, check the R4's routing table and you will see an IA route to 10.23.23.0 network. You can also verify with the show ip ospf virtual-links command.