We all know about OSPF (Open Shortest Path First) protocol–a fast link state protocol that is a go-to IGP (Interior Gateway Protocol) routing in large enterprises. It is also an open source protocol.

OSPF calculates shortest path for each route using Dijkstra’s algorithm. It uses area 0 as a backbone and every other area must be directly connected to it. There are cases in which we need to circumvent this rule and connect the OSPF area through the transit area to area 0. This occurs, for example, when the OSPF area has no direct connection to the backbone.

The configuration solution is an OSPF virtual link.

Let’s look at the following example:

R3 has no physical connection to the backbone, which means it cannot get routing updates. We will create an OSPF virtual link through R1 and R2 to connect R3 to area 0 by going through transit area AS100.

OSPF Configuration Virtual Link example:

Follow these steps to configure an OSPF virtual link:

• Configure OSPF

R1:

router ospf 1 log-adjacency-changes network 192.168.1.0 0.0.0.255 area 0 network 10.0.0.0 0.0.0.255 area 100

R2:

router ospf 1 log-adjacency-changes network 10.0.0.0 0.0.0.255 area 100 network 172.16.0.0 0.0.0.255 area 200

• Apply a virtual link with the following commands:

R1:

area 1 virtual-link 10.0.0.1

R2:

area 1 virtual-link 10.0.0.2

• To verify the virtual link, use the following command:

show ip ospf virtual-links

This is the output you should get:

R1#

show ip ospf virtual-links

Virtual Link OSPF_VL0 to router 10.0.0.2 is up

Run as demand circuit DoNotAge LSA allowed.

Transit area 100, via interface FastEthernet0/0, Cost of using 1 Transmit Delay is 1 sec, State POINT_TO_POINT,

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:06 Adjacency State FULL (Hello suppressed) Index 1/2, retransmission queue length 0, number of retransmision 0 First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0) Last retransmission scan length is 0, maximum is 0 Last retransmission scan time is 0 msec, maximum is 0 msec

That’s it! Now R3 will be able to receive routing updates from area 0 and you will have a converged network.