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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. Dear Teacher Rene,

    According to your lessons, OSPF Path Selection Explained , I already tested your topology.
    For O and OIA route no problem. your explanation and my lab result is same.
    After I shutdown loopback 0 interface on R3 (OIA route), On R1 I must seen OE1 for
    But I saw on on R4 is that, as an N1 route (not E1 route).
    It means After loopback0 on R3 shutdown, R1 more perfer N1 route on R6 rather than E1 route on R4.
    According to OSPF Path Selection, E1 should be seen as first and N1 is next to E1. Right??
    But If I play link cost increasing on R1 (connect to R6) as 1000, it change prefer E1 route than N1.

    I feel very confused.

    May you kindly teach and explain me more, please?
    Thank you very much indeed and Let us wait reply explanation from you, please.

  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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