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Figure 21 OSPF router types
Route types
OSPF prioritizes routes into the following route 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. The external routes
describe routes to external ASs.
A Type-1 external route has high credibility. The cost of a 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 has low credibility. OSPF considers that the cost from the ASBR to the
destination of a Type-2 external route is much greater than the cost from the ASBR to an OSPF
internal router. The cost of a Type-2 external route = the cost from the ASBR to the destination of the
Type-2 external route. If two Type-2 routes to the same destination have the same cost, OSPF takes
the cost from the router to the ASBR into consideration to determine the best route.
Route calculation
OSPF computes routes in an area as follows:
Each router generates LSAs based on the network topology around itself, and sends them to
other routers in update packets.
Each OSPF router collects LSAs from other routers to compose an LSDB. An LSA describes
the network topology around a router, and the LSDB describes the entire network topology of
the area.
Area 1
Area 2
Area 3
Area 4
Backbone router
ASBR
IS-IS
RIP
Internal router
ABR
Area 0
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