3com S7906E Manuel De Montage

 

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.  

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