How to Connect "Non Zero Area" to "area 0" via tunnelling in OSPF?

In multi-area OSPF network, all areas should be connected physically with area 0 (backbone area) for exchanging routing information and keeping routing table to date. Under certain cases, it is not possible to connect directly all areas in a network to backbone area. In this case, one can use virtual link to create a connection between backbone area and non backbone area. Virtual link is a logical connection to create virtual adjacency between two areas.

Two conditions must be fulfilled in order to configure a virtual link between different areas. 

• One area must be area 0 (backbone area).
• One area must be common between two endpoint routers to create virtual links.  

When we are talking about virtual links then another advanced form of virtual link is tunnelling. Tunnelling is normally being used in the complex network scenarios.

Lets take the example of following topology in Fig 1 in which consists of three routers. We can see how the router interfaces are connected to different areas. Routers R2 and R3 has interfaces which fall under the area 2 and they are not connected to the area 0.

Configuration of Tunnel

Configure and assign IP addresses to all of the router interfaces and configure them under the areas as depicted in the Fig 1.  

When we will run “show ip route” command on router R0, it is not displaying the route to 3.0.0.0 network, shown in the Fig 2.

Fig 2. Result of show IP route and ping to 3.1.1.1

On the other hand, the result of “show ip route” and ping to 1.1.1.2 can be seen on router R3 in the Fig 3. You will notice that show IP route is only displaying the 3.0.0.0 network.

Fig 3: Result of show IP route and ping to 1.1.1.2 on router R3

Configuration of Tunnel needs following series of commands.

R(config)# interface tunnel 0

R(config-if)# ip address {ip address} {net mask}

R(config-if)# tunnel source {source ip address}

R(config-if)#tunnel destination {destination ip address} 

Configure the OSPF for the network addresses of the tunnels.

Fig 4 and Fig 5 shows the configuration of tunnels on the routers R1 and R2 respectively.

Fig 4: Configuration of tunnel on R1

Fig 5 : Configuration of tunnel on R2

After the successful configuration of the tunnelling we will notice in Fig 6 that on the router R3, ping was successful to 1.1.1.2 and R3 was showing all other networks as well, previously it was displaying only 3.0.0.0 network.

Fig 6: Result of “Show ip route” and ping to 1.1.1.2 on router R3

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