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  1. Hi Rene,
    what is the default network type for DMVPN tunnel?

    If it is point-to-point, we must change it and become mandatory to change it?

  2. Mahmoud,
    You are exactly right. The default network type of a DMVPN tunnel is Point-to-Point.

    One of the rules of a P2P interface is there can be at most 1 OSPF neighbor. With the Hub’s tunnel interface, however, the OSPF process hears Hello messages from numerous different neighbors’ OSPF processes. This causes the Hub’s OSPF process to churn over and over, throwing out the previously formed Exstart relationship to form a new neighborship with the most recently received Hello. When this happens, you will see these messages on the Hub over and over again for each Spoke it has:
    OSPF-5-ADJCHG: Process 1, Nbr on Tunnel0 from EXSTART to DOWN, Neighbor Down: Adjacency forced to reset

    To fix this, you MUST change the network type of the tunnel. You have two choices here, Point-to-Multipoint or Broadcast. Which should you choose? If you pick the P2MP network type, DMVPN will not be able to function any more efficiently than at Phase 1. The reason for this is because the P2MP network type changes the next hop value of all traffic to be the hub. This means that all traffic flows through the hub, and you are no better off than Phase 1.

    Therefore, you should change the network type of the tunnel to be Broadcast. Even this, however, has its pitfalls. You must ensure that each spoke is configured so that it will never be elected as the DR or BDR via the “ip ospf priority 0” command.

    Can you see why it is generally recommended NOT to run OSPF via DMPVN? :slight_smile:

  3. Hey There,

    is it really required to change the OSPF priority to 0 on the spokes in case you go for OSPF network type Broadcast?
    (if i recall it right with DMVPN Ph2 you should have spoke-to-spoke direct connectivity and if this is true then it should be technically possible for the spokes to be elected as DR/BDR )


  4. Hello Salvatore

    Yes you are correct that for Phase 2 with a broadcast network type, there is direct spoke-to-spoke connectivity. However, this direct connectivity is achieved AFTER the initial communication occurs with the HUB. When a spoke router wants to reach another spoke, it will send an NHRP resolution request to the hub to find the NBMA IP address of the other spoke. This means that initial connectivity must be made with the hub router.

    Additionally, in order to achieve this direct spoke-to-spoke connectivity, you need two things:

    * Spoke routers need to have a route for the network that they are trying to reach.
    * The next hop IP address of the route has to be the remote spoke.

    So really in order to achieve spoke-to-spoke connectivity, routing between the spokes must first be established. Initial communication only occurs between the Hub and each individual spoke and this is where the routing exchange must occur.

    I hope this has been helpful.


  5. Ciao Lazaros,

    thanks for your detailed description,

    if i understood it right, it seems to be a matter of priorities:
    before the spoke-to-spoke communication can work I should have hub & spoke reachability which means that, in case I decide to use OSPF, the DR/BDR election it’s a prerequisite for the spoke-to-spoke communication to be established


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