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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. system says:

    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. maher1 says:

    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 tree remains unchanged. Incremental SPF allows the system to recompute only the affected part of the tree. Recomputing only a portion of the tree rather than the entire tree results in faster OSPF convergence and saves CPU resources. Note that if the change to a Type-1 or Type-2 LSA occurs in the calculating router itself, then the full SPT is performed.
    Incremental SPF is scheduled in the same way as the full SPF. Routers enabled with incremental SPF and routers not enabled with incremental SPF can function in the same internetwork.

    Now to enable ISPF, it is a quite simple command:

    router ospf 1

    So as a summary, ISPF can make the process of OSPF to go faster by saving CPU resources when a change happen to Type-1 or Type-2 LSA.

    Hope this could help.

  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.


  5. Hi,
    If the Route type is E2 then Forward Metric will kick in and for others Load Balancing happend .Thx


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