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.