HP 4800G Series Configuration Manual page 442

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.
Figure 1-6 OSPF router types
Area 1
Internal router
Route types
OSPF prioritize routes into four levels:
Intra-area route
Inter-area route
Type-1 external route
Type-2 external route
The intra-area and inter-area routes describe the network topology of the AS, while external routes
describe routes to destinations outside the AS.
OSPF classifies external routes into two types: Type-1 and Type-2. A Type-1 external route is an IGP
route, such as a RIP or static route, which has high credibility and whose cost is comparable with the
cost of an OSPF internal route. The cost from a router to the destination of the Type-1 external route=
the cost from the router to the corresponding ASBR+ the cost from the ASBR to the destination of the
external route.
A Type-2 external route is an EGP route, which has low credibility, so OSPF considers the cost from the
ASBR to the destination of the Type-2 external route is much greater than the cost from the ASBR to an
OSPF internal router. Therefore, the cost from the internal router to the destination of the Type-2
external route= the cost from the ASBR to the destination of the Type-2 external route. If two routes to
RIP
IS-IS
ASBR
Area 0
ABR
Area 2
1-7
Area 4
Backbone router
Area 3