You could issue either the ip ospf network non-broadcast command or the ip ospf network point-to-multipoint non-broadcast command to configure a router interface to use unicast packets to establish Open Shortest Path First (OSPF) neighbor relationships. These commands are useful for creating an OSPF network over a non-broadcast multiaccess (NBMA) network, such as a Frame Relay network, where broadcast and multicast traffic is not allowed. However, OSPF neighbor discovery relies on multicast traffic, so routers on an NBMA network cannot automatically discover neighbor routers. Therefore, manual configuration of neighbor routers with the neighbor command is required with NBMA networks. All OSPF traffic is then sent to the unicast IP addresses configured in the neighbor commands.
The ip ospf network broadcast command, the ip ospf network point-to-point command, and the ip ospf network point-to-multipoint command all use multicast packets to establish OSPF neighbor relationships. As a result, these OSPF network types do not require manual configuration of neighbor routers. OSPF uses multicast addresses 224.0.0.5 and 224.0.0.6 to send linkstate advertisements (LSAs) and hello packets.