Configuring OSPF in a Phase 2 DMVPN network

Cliffs Notes version (details in the DMVPN webinar):

  • Configure ip nhrp multicast-map for the hub on the spoke routers. Otherwise, the spokes will not send OSPF hellos to the hub.
  • Use dynamic NHRP multicast maps on the hub router, or the spokes will not receive its OSPF hellos.
  • Use broadcast network type on all routers.
You could use the non-broadcast network type and configure the neighbors manually, but that would just destroy the scalability of the solution. If you use point-to-multipoint network type, all the traffic will flow through the hub router.
  • Only the NHRP next-hop servers (usually the hub router(s)) can send OSPF hellos to everyone. If any other router becomes DR or BDR, you’ll get hard-to-explain partial connectivity.
  • Set OSPF DR priority to a high value on the hub router (so even if an under-configured spoke joins the network, it won’t preempt the hub).
  • Set OSPF DR priority to zero on the spoke routers (this ensures they can never become a DR or BDR).
  • The DMVPN part of your network should be a separate OSPF area; if at all possible, make it a stub or NSSA area.
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