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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 communication must go through it. All areas should be
physically connected to area 0 so the routing information injected
into this backbone can be disseminated to other areas.
Note The backbone area must always be configured as Area
0.
There are situations, however, when a new area is added after the
OSPF internetwork has been designed and configured 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, as shown in Figure . 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 the true cost to reach a destination
across the backbone can be calculated.
Virtual links serve the following purposes:
- Linking an area that does not have a physical connection to
the backbone. This linking could occur, for example, when two
organizations merge.
- Patching the backbone if discontinuity in area 0 occurs.
Figure
illustrates
the second purpose. For example, discontinuity of the backbone might
occur 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 via a virtual link through the common area 3. If a
common area does not already exist, one can be created to become the
transit area.
Note Area 0 could become partitioned, for example, if
two OSPF networks become merged.
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