OSPF Virtual Links

Virtual Link is a non-Physical connectivity or link. As we know OPSF must be connected with Backbone Area0 but sometime it becomes difficult due to some reasons. So there we required Virtual Links for logical connection to Backbone.

Also known as logical link which uses in least cost path connectivity between routers or ABR or backbone/Non-backbone. A virtual link does not have ABR physical Connectivity to Backbone till to configure virtual link between non-backbone area and Backbone. In between two routers, a virtual link can be configured, that have an interface to a single common Transit/Non-backbone area but it can’t happen in multiple transit/Non-backbone area. Transit/Non-backbone area keeps an ABR which is connected with Backbone area.

GRE tunnels can also serve the same purpose but it adds a lot of extra overhead when the network size grows. There are some other differences as well which are listed in below:

Configuration Example

Lets have this explained through an example. Consider below topology with three areas:

Here are three Areas shown in picture as Area 0, Area 1 & Area 2. Here Area 2 is not connected with Area 0 but it is possible by using Virtual Link that can extend Area 0 through Area 1, so Area 2 will be directly connected to Area 0. Let’s take a look at how a virtual link can solve this problem.

The following example configures a virtual link between two devices.

After this configuration, if everything is OK even Area2 is not directly connected to Backbone Area (Area0), it will still be connected to Area0 virtually. Let’s try to ping R1’s loopback from R3:

We can also use the #show ip ospf virtual-links command to check if our virtual-link is working:

R1#show ip ospf virtual-links
Virtual Link OSPF_VL0 to router 192.168.23.2 is up
Run as demand circuit
DoNotAge LSA allowed.
Transit area 1, via interface FastEthernet0/0, Cost of using 1