As described in RFC 2328, OSPF runs in one of the following two official modes in NBMA topologies:
1. nonbroadcast
2. point-to-multipoint
The nonbroadcast (NBMA) mode simulates the operation of OSPF in broadcast networks. Neighbors must be manually configured, and DR and BDR election is required. This configuration is typically used with fully meshed networks.
point-to-multipoint
The point-to-multipoint mode treats the nonbroadcast network as a collection of point-to-point links. In this environment, the routers automatically identify their neighboring routers but do not elect a DR and BDR. This configuration is typically used with partially meshed networks.
The choice between NBMA and point-to-multipoint modes determines the way the Hello protocol and flooding work over the nonbroadcast network. The main advantage of the point-to-multipoint mode is that it requires less manual configuration, and the main advantage of the nonbroadcast mode is that there is less overhead traffic.
So Point to multipoint option does not require neighbors to be manually defined as it is treated as a collection of P2P links.
It uses a single subnet to represent the network and uses multicast to discover the neighbors and does not require DR or BDR
Cisco has defined the following additional modes:
point-to-multipoint nonbroadcast
broadcast
point-to-point
HTH
Narayan
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