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| 6.8 |  | Virtual Links |
| 6.8.1 |  | Meeting the backbone area requirements |

OSPF has certain restrictions when multiple areas are configured. One area must be defined as Area 0, the backbone area. It is called the backbone because all inter-area communication must go through it. Therefore, all areas should be physically connected to Area 0 so that the routing information injected into this backbone can be disseminated to other areas. The backbone area must always be configured as Area 0. No other area ID can function as the backbone.

There are situations, however, when a new area is added after the OSPF internetwork has been designed, and it is not possible to provide that new area with direct access to the backbone. In these cases, a virtual link can be defined to provide the needed connectivity to the backbone area. The virtual link provides the disconnected area a logical path to the backbone. All areas must connect directly to the backbone area or through a transit area.

The virtual link has the following two requirements:

- It must be established between two routers that share a common area.
- One of these two routers must be connected to the backbone.

When virtual links are used, they require special processing during the SPF calculation. That is, the ’real’ next-hop router must be determined so that the true cost to reach a destination across the backbone can be calculated.

Virtual links serve the following purposes:

- They can link an area that does not have a physical connection to the backbone. This linking could occur, for example, when two organizations merge.
- They can patch the backbone if discontinuity in Area 0 occurs. Discontinuity of the backbone might occur, for example, if two companies merge their two separate OSPF networks into a single one with a common Area 0. The only alternative for the companies is to redesign the entire OSPF network and create a unified backbone.

Another reason for creating a virtual link is to add redundancy in cases when router failure might cause the backbone to be split into two.

In Figure , the disconnected Area 0s are linked through a virtual link through the common area, Area 3. If a common area does not already exist, one can be created to become the transit area. Area 0 could become partitioned, for example, if two OSPF networks were merged.

| Web Links Configuring OSPF http://www.cisco.com/en/US/products/ sw/iosswrel/ps1828/products_configuration_ guide_chapter09186a00800ca56c.html