3com S7906E Manuel De Montage

 

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In addition, as the topology of a large network is prone to changes, enormous OSPF packets may be 

created, reducing bandwidth utilization. Each topology change makes all routers perform route 

calculation. 

To solve this problem, OSPF splits an AS into multiple areas, which are identified by area ID. The 

boundaries between areas are routers rather than links. A network segment (or a link) can only reside in 

one area, in other words, an OSPF interface must be specified to belong to its attached area, as shown 

in the figure below. 

Figure 1-1 OSPF area partition 

 

 

After area partition, area border routers perform route summarization to reduce the number of LSAs 

advertised to other areas and minimize the effect of topology changes. 

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: 

z 

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

z 

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.