OSPF Virtual Link

A virtual link is a logical channel set up between two ABRs through a non-backbone area.

As required in RFC 2328, all non-backbone areas must be connected to the backbone area to ensure that all areas are reachable. However, it is not true in some scenarios due to certain restrictions. To resolve this issue, you can configure an OSPF virtual link.

In Figure 1, Area 2 is not connected to Area 0. Therefore, RouterA cannot function as an ABR to generate routing information of Network1 and advertise the information to Area 2, and RouterB does not have routes to Network1. A virtual link can be deployed to resolve this issue.

Figure 1 Non-backbone area not connected to the backbone area

In Figure 2, two ABRs use a virtual link to directly transmit OSPF packets. The OSPF device between the two ABRs only forwards packets. Because the destination of OSPF packets is not the OSPF device, the OSPF device transparently transmits the OSPF packets as common IP packets.

Figure 2 OSPF virtual link implementation principles

A virtual link functions as a point-to-point connection between two ABRs. The interfaces on both ends of the virtual link can be configured with parameters such as the Hello interval in the same way these parameters are configured on physical interfaces. The transmit area refers to the area that provides an internal route of a non-backbone area for both ends of the virtual link. A virtual link must be configured at both ends of the link, or it will not take effect.

A virtual link increases the network complexity and complicates fault locating. You are not advised to use virtual links during network planning. Virtual links are only a temporary measure to fix unavoidable network topology problems. When virtual links are designed, you need to consider whether to re-plan the network.