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Figure 19 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 from a router to the destination 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 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 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.
Area 1
Area 2
Area 3
Area 4
Backbone router
ASBR
IS-IS
RIP
Internal router
ABR
Area 0
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