OSPF - Adjacency Between NBMA and P2MP Non-Broadcast Networks
OSPF adjacencies do not automatically form between NBMA (Non-Broadcast Multi-Access) and P2MP (Point-to-Multipoint) Non-Broadcast networks because these network types do not send hello packets to the multicast address. However, configuring the neighbor command on one of the routers will enable the adjacency to form. This is achieved by the router with the neighbor command sending unicast hello packets to the specified neighbor, which then responds with unicast hellos, establishing the adjacency.