How to configure virtual link between different areas in OSPF

How to configure virtual link between different areas in OSPF?

In multi-area OSPF protocol network, all areas should be connected physically with area 0 (backbone area) for exchanging routing information. But in some cases, it is not possible to connect directly all areas in a network to backbone area. In this case, you can use virtual link to create a connection between backbone area and non backbone area. Basically virtual link is a logical connection to create virtual adjacency between two areas using the least cost path.

Two conditions must be fulfilled to apply a virtual link between different areas.

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

Follow the network topology of three different class network in given figure:

There are some common OSPF configuration and Executable commands:

OSPF configuration on Router A

In global configuration mode

Router-A(config) # router ospf 1 (Here 1 indicate the process identification number)

Router-A(config-Router) # network 210.100.10.0 0.0.0.255 area 0 (Here 210.100.10.0 indicate the network ID and 0.0.0.255 wildcard mask with area 0)

Router-A(config-Router) # network 20.0.0.0 0.255.255.255 area 1 (Here 20.0.0.0 indicate the network ID and 0.225.255.255 wildcard mask with area 1)

Router-A(config-Router) #area 1 virtual-link 31.1.1.1 (Here 1 indicate the area ID and 31.1.1.1 is the highest loopback address of router B)

Router-A(config-Router) #exit (exit from ospf 1 on router A)

OSPF configuration on Router B

In global configuration mode

Router-B(config) # router ospf 1 (Here 1 indicate the process identification number)

Router-B(config-Router) # network 210.100.20.0 0.0.0.255 area 1 (Here 210.100.20.0 indicate the network ID and 0.0.0.255 wildcard mask with area 1)

Router-B(config-Router) # network 20.0.0.0 0.255.255.255 area 1 (Here 20.0.0.0 indicate the network ID and 0.225.255.255 wildcard mask with area 1)

Router-B(config-Router) # network 30.0.0.0 0.255.255.255 area 2 (Here 30.0.0.0 indicate the network ID and 0.225.255.255 wildcard mask with area 2)

Router-B(config-Router) #area 1 virtual-link 21.1.1.1 (Here 1 indicate the area ID and 21.1.1.1 is the highest loopback address of router A)

Router-B(config-Router) #exit (exit from ospf 1 on router B)

OSPF configuration on Router C

In global configuration mode

Router-C(config) # router ospf 2 (Here 2 indicate the process identification number)

Router-C(config-Router) # network 210.100.30.0 0.0.0.255 area 2 (Here 210.100.30.0 indicate the network ID and 0.0.0.255 wildcard mask with area 2)

Router-C(config-Router) # network 30.0.0.0 0.255.255.255 area 2 (Here 30.0.0.0 indicate the network ID and 0.225.255.255 wildcard mask with area 2)

Router-C(config-Router) #exit (exit from ospf 1 on router C)

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