3Com 5500-EI PWR Install Manual page 291

Figure 4-1 OSPF area partition
On the border of an area is a router, which belongs to different areas. 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.
4) Autonomous system border router (ASBR)
The router exchanging routing information with another AS is an ASBR, which may not reside on the
boundary of the AS. It can be an internal router or area border router.
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