What is Virtual-link?

OSPF virtual link is a virtual area 0 point-to-point link. Virtual link is configured between two ABRs. And the area that the two ABRs that are configuring the virtual link belong to in common is called transit area.

Virtual-link is used in the following cases.

  • Virtually connect to area 0
  • Integrating Discontinuous Backbone Areas

In the following sections, we will discuss the use of virtual-link.

Usage of Virtual Link : Virtually connect to area 0

The OSPF area must be in two layers, with the backbone area (area 0) at the center. All areas should be connected to area 0. It is an illegal area layout to connect more areas beyond the area other than area 0.

Figure Illegal Area Layout
Figure Illegal Area Layout

If you are forced to take a illegal area layout where there are areas that cannot be physically connected to Area 0, you can use virtual-link to virtually connect to Area 0.

The following figure shows a virtual connection to Area 0 via a virtual-link.

Figure Connecting to Area 0 virtually with a virtual-link
Figure Connecting to Area 0 virtually with a virtual-link

Area 3 is an area that cannot be physically connected to Area 0. The route information of the networks in Area 3 that are not connected to the backbone area will not be advertised to the OSPF domain. Virtual-link is used to ensure that the route information of Area 3 networks can be learned successfully.

Configure a virtual link between ABR1 and ABR2 in the figure; Area 1, to which ABR1 and ABR2 commonly belong, is the transit area of the virtual-link. Then ABR1 and ABR2 will be connected by a virtual area 0 point-to-point link. In other words, Area 0 is extended and Area 3 can be considered as connected to Area 0.

However, the use of virtual-link in such an area layout should be considered as a temporary solution. Eventually, it is better to integrate Area 3 into Area 1 so that the virtual link is not needed.

Usage of Virtual Link : Integrating Discontinuous Backbone Areas

An area layout in which multiple backbone areas (area 0) exist in discontinuity is also an illegal area layout.

Figure Discontinuous backbone
Figure Discontinuous backbone

If these backbone areas are discontinuous, communication will be possible in a two-tier area centered on each backbone area. However, the entire OSPF network will not be able to communicate. There is no problem between the two tiered areas with the backbone area on the left. Area 1-Area 0 (left)-Area 2 is reachable. Also, Area 2 – Area 0 (right) – Area 3 around the backbone area on the right can be communicated. However, communication between Area 1 and Area 3 is not possible. Also, communication between Area 0, which is divided, is not possible.

Figure Range of communication possible with discontinuous backbone Part 1
Figure Range of communication possible with discontinuous backbone Part 1
Figure Range of communication possible with discontinuous backbone Part 2
Figure Range of communication possible with discontinuous backbone Part 2

Virtual-link allows integration of discontinuous backbone areas so that they can communicate across the entire OSPF domain.

Figure Integrating Discontinuous Backbones with Virtual-Link
Figure Integrating Discontinuous Backbones with Virtual-Link

Summary

Point

  • Virtual link is a virtual area 0 point-to-point link.
  • Virtual link is configured between two ABRs that belong to a common transit area.
  • Usage of virtual-link is the following
    • Virtually connect to area 0
    • Integrating Discontinuous Backbone Areas

Related article

The following article explains the configuration and verification commands for OSPF virtual-link on Cisco routers.

N-Study
Configuring and Verifying OSPF Virtual-link [Cisco] | N-Study
https://www.n-study.com/en/ospf-detail/cisco-configuring-ospf-virtual-link/
This section describes the configuration and verification commands for OSPF virtual-link on Cisco routers.

How the OSPF works