Rohitendu,
Cisco has five different options for OSPF over NBMA. Which one you should choose depends on several factors. One important consideration is whether the circuit supports multicast–this will determine whether you must define neighbors manually, or if they can discover each other. Additionally, you need to consider what is the topology of your WAN connection–full mesh, hub and spoke, …? This is important because you must ensure that any router eligible to be a DR or BDR must have reachability to all the other OSPF nodes.
Here is some information on the five different OSPF NBMA network types:
5 Modes of Operation
-Non-Broadcast (NBMA) - RFC Standard
-Point-To-Multipoint - RFC Standard
-Point-To-Point - Cisco Proprietary
-Broadcast - Cisco Proprietary
-Point-To-Multipoint, Non-Broadcast - Cisco Proprietary
Broadcast Mode
-LAN-style operational mode over NBMA (there is no RFC standard for this, hence Cisco proprietary)
-(config-if)# ip ospf network broadcast
Non-Broadcast Mode
-Default mode for X.25, FR, ATM
-Neighbors are statically configured
-Must be on one subnet
-Acts like a LAN environment
-DR/BDR are elected (DR/BDR must have full connectivity to all neighbors). This point will be a design constraint as your only option for a DR/BDR in a hub and spoke topology would be the hub
-Typically used in full mesh networks
Point-To-Multipoint Mode
-Requires a single subnet
-No DR/BDR elected
-Neighbors form automatically as long as the “broadcast” option is chosen in configuring the link
-Typically used in partial mesh networks
-(config-if)# ip ospf network point-to-multipoint
Point-To-Multipoint, Non-Broadcast
-Exactly the same as P2P mode, but neighbors do not form automatically. They must be configured manually
Point-To-Point Mode
-Uses separate sub-interfaces
-Requires different subnets
-No DR/BDR elected
-Neighbors form automatically