Non-broadcast multi-access (NBMA) network – A type of network that allows for multi-access but does not have broadcast ability (i.e. Frame Relay, X.25 and ATM networks) . NBMA networks require special OSPF configuration to function properly and it eliminates the need for DR and BDR.
there are 5 modes:
1.RFC NBMA - fully meshed, same subnet, manual config DR/BDR
2.Cisco Broadcast - fully meshed, same subnet, Auto config DR/BDR.
3.RFC Point-to-Multipoint - partial meshed/Star, same subnet, auto config DR/BDR.
4.Cisco Point-to-Multipoint - partial meshed/Star, same subnet, no DR/BDR elected.
5.Cisco Point-to-Point - partial meshed/Star, Diff subnet, no DR/BDR elected.