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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 has the following two requirements:
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:
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
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