OSPF virtual link

FIGURE 6 . 8 OSPF virtual link
The syntax for creating a virtual link across an area is
area area-id virtual-link router-id
where area-id is the number of the transit area, in this example Area 10, and router-id is the
IP address of the highest loopback interface configured on a router or can be manually set.
To manually set the router-id, you need to enter the command router-id id-in-IPaddress-
format in router configuration mode. If you do not manually set the router-id and
a loopback interface has not been configured on the router, then the router-id is the highest
IP address configured on the router.
Note that a virtual link has area border routers as the end points of the link. Virtual links will
be used, in the real world, to merge corporate networks during transition phases. When two
companies merge and the companies both have OSPF networks, both companies will have their
own Area 0. In this instance, a virtual link would be created between the discontiguous Area 0s
until the networks could be fully merged.
As shown in Figure 6.9, we are going to create a virtual link from Area 20 to Area 0, with
Area 10 acting as the transit area. Let’s examine the configuration of RouterB and RouterC,
because RouterA does not have any virtual-link-specific configuration.
Here are the configurations of RouterB and RouterC:
RouterB(config)#router ospf 10
RouterB(config-router)#network 3.0.0.0 0.255.255.255 area 0
RouterB(config-router)#network 4.0.0.0 0.255.255.255 area 10
RouterB(config-router)#area 10 virtual-link 6.6.6.1
RouterA
RouterB RouterC
Ethernet Ethernet
Lo0:2.2.2.1/24
1.1.1.1/24
e0 e1
e0
e1
3.3.3.1/24
e0
e1
3.3.3.2/24
4.4.4.1/24
4.4.4.2/24
7.7.7.1/24
Area 0 Area 10 Area 20
Lo0:5.5.5.1/24 Lo0:6.6.6.1/24
194 Chapter 6  Interconnecting OSPF Areas