OSPF Virtual Links

Today I am going to talk about OSPF Virtual Links. In order to understand the OSPF virtual links, first you guys make sure all areas used in an OSPF AS must be physically connected to the backbone area (area 0). Area 0 is the main area where all area's need to connect physically. 

In some cases where this physical connection is not possible, you can use a virtual link to connect to the backbone through a non-backbone area. You can also use virtual links to connect two parts of a partitioned backbone through a non-backbone area. The area through which you configure the virtual link, known as a transit area, must have full routing information. The transit area cannot be a stub area.

Use the area area-id virtual-link router-id command in order to configure a virtual link, where the area-id is the area ID assigned to the transit area (this can be either a valid IP address or a decimal value), and where router-id is the router ID associated with the virtual link neighbor. In this example, the virtual link connects area 7 to the backbone through area 5

Fig 1.1- OSPF Virtual Links

In this example, the virtual link is created between the routers with router ID 1.1.1.1 and router ID 2.2.2.2. In order to create the virtual link, configure the area 5 virtual-link 2.2.2.2 subcommand on router 1.1.1.1 and the area 5 virtual-link 1.1.1.1 subcommand on router 2.2.2.2.