H3C S3610-28P Operation Manual page 374

Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
Figure 3-2 OSPF router types
III. Backbone area and virtual links
Each AS has a backbone area, which is responsible for distributing routing information
between none-backbone areas. Routing information between non-backbone areas
must be forwarded by the backbone area. Therefore, OSPF requires that:
All non-backbone areas must maintain connectivity to the backbone area.
The backbone area itself must maintain connectivity.
In practice, due to physical limitations, the requirements may not be satisfied. In this
case, configuring OSPF virtual links is a solution.
A virtual link is established between two area border routers via a non-backbone area
and is configured on both ABRs to take effect. The area that provides the
non-backbone area internal route for the virtual link is a "transit area".
In the following figure, Area 2 has no direct physical link to the backbone area 0.
Configuring a virtual link between ABRs can connect Area 2 to the backbone area.
Figure 3-3 Virtual link application 1
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Chapter 3 OSPF Configuration