86
•
Compared with a stub area, an NSSA area can import external routes through Type-7 LSAs
advertised by the ASBR.
•
Compared with an NSSA area, a totally NSSA area does not import inter-area routes.
Router types
Classification of routers
The following are OSPF router types and their positions in the AS:
•
Internal router: All interfaces on an internal router belong to one OSPF area.
•
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.
•
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.
•
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 23
OSPF router types
Area 1
Area 2
Area 3
Area 4
Backbone router
ASBR
IS-IS
RIP
Internal router
ABR
Area 0
Route types
OSPF prioritizes routes into the following levels:
•
Intra-area route
