Virtual Links

It is possible to define areas in such a way that they do not have an ABR physically connected to the backbone. Backbone connectivity for the area is still possible by configuring a virtual link between the non-backbone area and the backbone.

Virtual links can be configured between any two routers that have an interface to a single common non-backbone area. The common non-backbone area is known as the transit area. The transit area must have an ABR that is connected to the backbone. Virtual links cannot be configured across multiple transit areas.

A virtual link is not a physical link. It is a logical link using the least cost path between the ABR of the non-backbone connected area and the backbone ABR of the transit area. A virtual adjacency across the virtual link is formed, and routing information is exchanged. Just as in physical adjacencies, the settings of the two virtual link routers (such as the password, the Hello Interval, and Dead Interval) must match before an adjacency can be successfully established.

In Figure 3.20, Area 0.0.0.3 does not have a router physically connected to the backbone, Area 0.0.0.0. Therefore, a virtual link is configured across the transit area of Area 0.0.0.2 between routers R2 and R3. R2 and R3 are known as virtual link neighbors.

Enlarge figure

Figure 3.20 OSPF Virtual Link

Configuring Virtual Links

To configure a virtual link, configure a virtual interface in the Routing and Remote Access snap-in from the properties of the OSPF routing protocol on each virtual link neighbor with the following:

• The Area ID of the transit area for the virtual link.
• The OSPF Router ID of the virtual link neighbor.
• Adjacency settings such as the Retransmit Interval, Hello Interval, Dead Interval, and Password. The Hello Interval, Dead Interval, and Password must match between the two routers on each side of the virtual link in order for an adjacency to be established. The Retransmit Interval specifies how long the OSPF router waits before retransmitting Link State packets.

To view the virtual links

• In the Routing and Remote Access snap-in, in the IP Routing container, right-click OSPF, and then click Show Virtual Interfaces.

  The Virtual Interfaces window displays all configured virtual interfaces and their state.

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