WB1 6.12 Repairing Discontiguous OSPF Areas with Virtual-Links

• Configure the network so that full reachability is maintained if R3’s connection to SW1 goes down.
• Do not apply any configuration onto R4 to solve this task.

若是R3的Fa0/0與SW1之間斷掉，那 Area 2(SW3、SW1、R6)間就沒有任何路由器跟Area 0介接。

只有在R6與R1間，透過Area 1與Area 0介接。

R1:
router ospf 1
  area 1 virtual-link 150.1.6.6

R6:
router ospf 1
  area 1 virtual-link 150.1.1.1

To resolve this problem R6 must offer devices in area 2 an alternate path to area 0. This can be done with a new link being added to R6 that is in area 0, such as another Ethernet interface or a Tunnel interface, or by configuring an OSPF virtual-link.

An OSPF virtual-link allows the creation of an indirect area 0 adjacency. This adjacency can be used to repair breaks in the OSPF domain or to solve traffic engineering requirements. In this particular case R6 can virtual-link to the neighbors R1 or R4 who are in area 1, since they both have connections to area 0. R1 is chosen in this solution since the question states not to configure R4.

The first important point to note about the virtual-link is that the neighbor value specified in the virtual-link syntax is the router-id of the neighbor in the transit area. This means that if for some reason the router-id changes, i.e. a new higher Loopback interface is added, or the router-id command is changed, the virtual-link will fail.

Additionally, the neighbors forming adjacency over the virtual-link do not have to be directly connected; they simply need to know how to recurse towards each others’ LSA 1 advertisements. This means that the traffic flow via the virtual-link should naturally follow the Intra-Area SPF calculation between the routers’ LSA 1 advertisements.