Backbone area and virtual links – H3C Technologies H3C S5560 Series Switches User Manual

Page 81

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Figure 15 Area-based OSPF network partition

Backbone area and virtual links

Each AS has a backbone area that distributes routing information between non-backbone areas. Routing

information between non-backbone areas must be forwarded by the backbone area. OSPF has the

following requirements:

All non-backbone areas must maintain connectivity to the backbone area.

The backbone area must maintain connectivity within itself.

In practice, these requirements might not be met due to lack of physical links. OSPF virtual links can solve

this issue.
A virtual link is established between two ABRs through a non-backbone area. It must be configured on

both ABRs to take effect. The non-backbone area is called a transit area.
As shown in

Figure 16

, Area 2 has no direct physical link to the backbone Area 0. You can configure a

virtual link between the two ABRs to connect Area 2 to the backbone area.

Figure 16 Virtual link application 1

Virtual links can also be used to provide redundant links. If the backbone area cannot maintain internal
connectivity because of the failure of a physical link, you can configure a virtual link to replace the failed

physical link, as shown in

Figure 17

.

Area 0

Area 1

Area 2

Area 3

Area 4

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