Hello Hosam
DMVPN is a network topology that has a particular behaviour. Specifically, DMVPN is a NBMA network topology, similar to Frame-Relay. Even so, OSPF considers any GRE multipoint tunnel interface as point to point. If you configure OSPF without any designation of network types, the default will be point to point. You can verify this by creating a topology, configuring OSPF without any network type configuration, and then issue the show ip ospf int tunnel 1, you’ll see the following:
Tunnel1 is up, line protocol is up
Internet Address 10.255.253.10/24, Area 0
Process ID 100, Router ID 10.10.10.10, Network Type POINT_TO_POINT, Cost: 11111
Note the Network Type information. Unless you change this to an appropriate configuration, it will simply not work.
If you choose to use broadcast, then OSPF will function just like any Ethernet-based network will function by default. All three routers, which are in the same subnet on their tunnel interfaces, will learn each other’s routes, and neighbor adjacencies will form (assuming you’ve configured the hub to be the DR, as per the lesson.)
If you choose non-broadcast, then the result is exactly the same, except for the fact that neighbor adjacencies will not form automatically, but must be manually configured, as stated in the lesson.
I hope this has been helpful!
Laz