3Com Switch 4800G 24-Port Configuration Manual page 276

276 C 28: OSPF C
HAPTER
OSPF Area Partition and
Route Summarization
ONFIGURATION
Area partition
When a large number of OSPF routers are present on a network, LSDBs may
become so large that a great amount of storage space is occupied and CPU
resources are exhausted by performing SPF computation.
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 73 OSPF area partition
Area 1
Area 2
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.
Classification of Routers
The OSPF routers fall into four types according to the position in the AS:
1 Internal Router
All interfaces on an internal router belong to one OSPF area.
2 Area Border Router (ABR)
An area border router belongs to more than two areas, one of which must be the
backbone area. It connects the backbone area to a non-backbone area. The
connection between an area border router and the backbone area can be physical
or logical.
Area 4
Area 0
Area 3