H3C S5600 Series Operation Manual page 355

Operation Manual – Routing Protocol
H3C S5600 Series Ethernet Switches
III. Backbone area and virtual link
1) Backbone area
With OSPF area partition, not all areas are equal. One of the areas is different from
any other area. Its area ID is 0 and it is usually called the backbone area. Routing
information between non-backbone areas must be forwarded by the backbone area.
Therefore, OSPF requires:
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.
2) Virtual link
A virtual link is established between two area border routers through 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 4-3 Virtual link application 1
Another application of virtual links is to provide redundant links. If the backbone area
cannot maintain internal connectivity due to a physical link failure, configuring a virtual
link can guarantee logical connectivity in the backbone area, as shown below.
Figure 4-4 Virtual link application 2
4-7
Chapter 4 OSPF Configuration