H3C Technologies H3C S12500 Series Switches - Manual (Page 73)

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Adjacency—Two OSPF neighbors establish an adjacency relationship to synchronize their LSDBs. Any
two neighbors not exchanging route information will not establish an adjacency.
OSPF areas
Network partition
In a large OSPF routing domain, SPF route computations consume too many storage and CPU resources,
and enormous OSPF packets generated for route synchronization occupy excessive bandwidth.
To solve these problems, OSPF splits an AS into multiple areas. Each area is identified by an area ID. The
boundaries between areas are routers rather than links. A network segment (or a link) can only reside in
one area, as shown in Figure 17 .
You can configure route summarization on ABRs to reduce the number of LSAs advertised to other areas
and minimize the effect of topology changes.
Figure 17 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. Therefore, OSPF
requires the following:
• All non-backbone areas must maintain connectivity to the backbone area.
• The backbone area itself must maintain connectivity.
In practice, the requirements might not be met due to lack of physical links. OSPF virtual links can solve
this problem.
A virtual link is established between two ABRs through a non-backbone area and is configured on both
ABRs to take effect. The non-backbone area is called a transit area.
In
Figure 18 , 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.
Area 0
Area 1
Area 2
Area 3
Area 4