1-7z 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 LSAsadvertised by the ASBR.z Compared with an NSSA area, a totally NSSA area does not import inter-area routes.Router TypesClassification of RoutersThe OSPF routers fall into four types according to their positions in the AS:1) Internal RouterAll 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. Itconnects the backbone area to a non-backbone area. The connection between an area border routerand the backbone area can be physical or logical.3) Backbone RouterAt least one interface of a backbone router must be attached to the backbone area. Therefore, all ABRsand 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 theboundary of the AS. It can be an internal router or an area border router.Figure 1-6 OSPF router typesArea 1Area 2Area 3Area 4Backbone routerASBRIS-ISRIPInternal routerABRArea 0
