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Forum Replies

  1. Great post Rene. OSPF is a protocol with lots of details. We can see some interesting things when we do OSPF external type 2 with same cost but different cost to ASBR.

  2. First of all thankx to all,
    my ques.
    If there are multiple routes to a network with the same route type and cost,what is the next criteria 4 selecting best path.

  3. Hi Brandon,

    That’s a good question and rarely asked.

    Here the best explanation that is taken from Cisco website about ISPF:

    As you know, OSPF uses Dijkstra’s SPF algorithm to compute the shortest path tree (SPT). During the computation of the SPT, the shortest path to each node is discovered. The topology tree is used to populate the routing table with routes to IP networks. When changes to a Type-1 or Type-2 link-state advertisement (LSA) occur in an area, the entire SPT is recomputed. In many cases, the entire SPT need not be recomputed because most of the tr

    ... Continue reading in our forum

  4. Hi Shaun,

    I believe the lowest OSPF Process ID is the tiebreaker. If you run two processes and both learn the same prefix with the same metric, the one with the lowest process ID should be installed in the routing table.

    Rene

  5. Hello Imen,

    OSPF doesn’t make a difference between inter-area or intra-area external routes. It doesn’t matter where the external route originated, it’s the cost that makes the difference.

    When I booted up this lab again, I did find something interesting. When I wrote this lesson, I used IOS 12.4 and this is the list that OSPF uses for path selection:

    • Intra-Area (O)
    • Inter-Area (O IA)
    • External Type 1 (E1)
    • NSSA Type 1 (N1)
    • External Type 2 (E2)
    • NSSA Type 2 (N2)

    Since Cisco IOS release 15.1(2)S, Cisco uses the path selection order from RFC 3101 which obsoletes RF

    ... Continue reading in our forum

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