12.1.5 Virtual link

In OSPF, two area border routers on areas that are not configured as stub areas or NSSA and are not backbones can be used as interfaces for the backbone by virtualizing the route between the two routers on this area with a point-to-point line. This virtual line is called a virtual link. The area that carries its routes is called the transit area.

The use of virtual links is described based on the following three examples:

Providing virtual connections for areas not physically connected to the backbone
Joining multiple backbones
Adding redundant connectivity to prevent faults from partitioning the backbone

(a) Virtual connections in areas that are not physically connected to the backbone

In the figure below, Area 2 is not connected to the backbone. If you configure a virtual link between Router 1 and Router 2 that uses Area 1 as its transit area, Router 2 considers itself an area border router because it has a connection to the backbone. Area 2 can now connect to the backbone through Router 2.

Figure 12-3: Connecting an area to the backbone

(b) Joining Multiple Backbones

The figure below shows an AS containing two backbone areas. This partition in the backbone can cause some destinations to become unreachable. You can avoid this problem by configuring a virtual link between Router 1 and Router 2 that uses Area 1 as its transit area to join the backbones.

Figure 12-4: Connections between backbones

(c) Preliminary pathway for disruption due to backbone failure

In the figure below, the connection between Router 1 and Router 2 is lost when a network fault occurs in the backbone, causing the backbone to be partitioned. In this case, if you configure a virtual link between Router 1 and Router 2 that uses Area 1 as the transit area, this becomes a spare route to the fragmentation of the backbone (the main route if the cost of the virtual link is sufficiently small compared with the cost of Router 1-Router 2 at the backbone).

Figure 12-5: Reserved paths for backbone partitioning