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Forum Replies

  1. 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

  2. Hi Rene, Hi all

    I still have problems for the choice between an E1 or N1 route (same between E2 or N2)
    In the drawing of this course, what would have been the result if the router injecting the E1 route had been an internal router of area 3 ? R1 would then have received :

    • an E1 route from that router, ie an inter-area external route of type E1
    • and an N1 route from area 167. But since R1 has an interface in that area, that would have been an intra-area external route

    Will R1 still prefer the E1 route, an inter-area external over an intra-area external ?

    Thanks a lot


  3. 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

  4. I apologize if this was already asked and answered, I just could not find it. So the questions is what is the criteria OSPF uses when choosing route between several equal cost and equal type paths. Thats in a single OSPF process, if one uses CISCO router, then CISCO by default set max path for OSPF to 4. But CISCO is not the only router there is and whatever is its default its CISCO default, not OSPF, strictly speaking. So how the OSPF itself then chooses the route to use? In other words, if I run max path 1 on the router (or use some router that has it as defa

    ... Continue reading in our forum

  5. Hello Rene/Laz,
    I am going to use the below topology for my question.


    In this diagram, OSPF is running everywhere. Router 3 and Router 4 have loopback addresses as it is depicted in the topology. My question is regarding path manipulation in OSPF. In this scenario, Router 4 is learning route through two different ways. However, Router 4 is installing route with the exit interface of G 0/2 as the best path (note: all the interfaces are Gig in

    ... Continue reading in our forum

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