3Com MSR 50 Series, Configuration Manual

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920 C
HAPTER
58: OSPF C
ONFIGURATION
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 269 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.
Classification of Routers
The OSPF router falls 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.
3
Backbone Router
At least one interface of a backbone router must be attached to the backbone
area. Therefore, all ABRs and internal routers in area 0 are backbone routers.
Area 0
Area 1
Area 2
Area 3
Area 4