When I read Cisco CCNP official guide, I found an explanation on how to calculate the cost of intra-area routes:

  1. Find all networks inside the area, based on the stub interfaces listed in the Type 1 LSAs and based on any Type 2 Network LSAs.
  2. Run SPF to find all possible paths through the area’s topology, from itself to each network.
  3. Calculate the OSPF interface costs for all outgoing interfaces in each route, picking the lowest-total-cost route for each network as the best route.

My question is: is finding all possible paths one of SPF algorithm's functions? I thought all possible paths is implicitly shown by LSDB, SPF algorithm's mission is just creating a SPF tree, and get the shortest path to each network which are leaves of this tree. Or here the author want to express the meaning that SPF traverses all possible paths and find the shortest path.

1 Answer 1


Each router in an area will run Dijkstra's (SPF) algorithm when its LSDB is updated to determine the shortest (lowest-cost) path for each network in the LSDB. The wording may be confusing. Every possible path on every possible exit point is examined to determine the lowest cost interface for the route. Each possible path must be walked to add up all the costs for the possible paths, and that may lead to the confusion. The SPF algorithm will look at each possible path to determine which has the lowest cost, and the interface toward that path for the route gets installed into the routing table for that route.

  • I agree with your point. Finding all possible paths is not the purpose but a procedure needs to be done to realize the real purpose. Thanks.
    – microbit
    Commented Jan 24, 2017 at 4:07

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