Configuring a virtual link

The OSPF backbone area, area 0.0.0.0, must be contiguous and all other areas must be connected to the backbone area. The backbone distributes routing information between areas. If it is not practical to connect an area to the backbone (see Area 0.0.0.5 in Figure: OSPF areas) then the area border routers (such as routers Y and Z) must be connected via a virtual link. The two area border routers form a point-to-point-like adjacency across the transit area (see Area 0.0.0.4). A virtual link can only be configured while in the area 0.0.0.0 context.

The router-id parameter specified in the virtual-link command must be associated with the virtual neighbor, that is, enter the virtual neighbor’s router ID, not the local router ID. The transit area cannot be a stub area or an NSSA.

Use the following CLI syntax to configure stub areas.

CLI syntax

ospf ospf-instance
    area area-id
  virtual-link router-id transit-area area-id
    authentication-key [authentication-key|hash-key] [hash]
    authentication-type [password|message-digest]
    dead-interval seconds
    hello-interval seconds
    message-digest-key key-id md5 [key|hash-key] [hash|hash2|custom]
    retransmit-interval seconds
    transit-delay
    no shutdown

The following displays a virtual link configuration example:

A:ALA-49>config>router>ospf# info
----------------------------------------------
            asbr
            overload
            overload-on-boot timeout 60
            traffic-engineering
            export "OSPF-Export"
            exit
            area 0.0.0.0
                virtual-link 1.2.3.4 transit-area 1.2.3.4
                    hello-interval 9
                    dead-interval 40
                exit
            exit
            area 0.0.0.20
                stub
                exit
            exit
            area 0.0.0.25
                nssa
                exit
            exit
            area 1.2.3.4
            exit
----------------------------------------------
A:ALA-49>config>router>ospf#

The following displays an OSPF3 virtual link configuration example:

A:ALA-48>config>router>ospf3# info
----------------------------------------------
            asbr
            overload
            timers
                lsa-arrival 50000
            exit
            export "OSPF-Export"
            area 0.0.0.0
                virtual-link 4.3.2.1 transit-area 4.3.2.1
                exit
            exit
            area 0.0.0.20
                stub
                exit
            exit
            area 0.0.0.25
                nssa
                exit
            exit
            area 4.3.2.1
            exit
----------------------------------------------
A:ALA-48>config>router>ospf3#