3Com S7906E - Switch, Configuration Manual

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1-7
z
Compared with a totally stub area, a stub area can import inter-area routes.
z
Compared with a stub area, an NSSA area can import external routes through Type 7 LSAs
advertised by the ASBR.
z
Compared with an NSSA area, a totally NSSA area does not import inter-area routes.
Router Types
Classification of Routers
The OSPF routers fall into four types according to their positions 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)
A 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 an area border router.
Figure 1-6 OSPF router types
Area 1
Area 2
Area 3
Area 4
Backbone router
ASBR
IS-IS
RIP
Internal router
ABR
Area 0