Connected: An Internet Encyclopedia
15. Virtual Links

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15. Virtual Links

15. Virtual Links

The single backbone area (Area ID = 0.0.0.0) cannot be disconnected, or some areas of the Autonomous System will become unreachable. To establish/maintain connectivity of the backbone, virtual links can be configured through non-backbone areas. Virtual links serve to connect physically separate components of the backbone. The two endpoints of a virtual link are area border routers. The virtual link must be configured in both routers. The configuration information in each router consists of the other virtual endpoint (the other area border router), and the non-backbone area the two routers have in common (called the transit area). Virtual links cannot be configured through stub areas (see Section 3.6).

The virtual link is treated as if it were an unnumbered point-to- point network (belonging to the backbone) joining the two area border routers. An attempt is made to establish an adjacency over the virtual link. When this adjacency is established, the virtual link will be included in backbone router links advertisements, and OSPF packets pertaining to the backbone area will flow over the adjacency. Such an adjacency has been referred to in this document as a "virtual adjacency".

In each endpoint router, the cost and viability of the virtual link is discovered by examining the routing table entry for the other endpoint router. (The entry's associated area must be the configured transit area). Actually, there may be a separate routing table entry for each Type of Service. These are called the virtual link's corresponding routing table entries. The InterfaceUp event occurs for a virtual link when its corresponding TOS 0 routing table entry becomes reachable. Conversely, the InterfaceDown event occurs when its TOS 0 routing table entry becomes unreachable.[19] In other words, the virtual link's viability is determined by the existence of an intra-area path, through the transit area, between the two endpoints. Note that a virtual link whose underlying path has cost greater than hexadecimal 0xffff (the maximum size of an interface cost in a router links advertisement) should be considered inoperational (i.e., treated the same as if the path did not exist).

The other details concerning virtual links are as follows:


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Connected: An Internet Encyclopedia
15. Virtual Links