OSPF Non-Broadcast Network Type over Frame-Relay

Hello Sam

Yes, you’re correct. Even in an NBMA environment, OSPF routers will still listen for hello packets on the multicast address. The difference lies in how they respond.

In a broadcast network type, OSPF routers will send hello packets to the multicast address to discover OSPF neighbors and form adjacencies.

However, in an NBMA network, routers won’t send hello packets to the multicast address due to the non-broadcast nature of the network. Instead, they expect to be configured with the neighbor command to manually specify neighbors, and unicast hello packets directly to those neighbors.

In your case, the adjacency still formed because the broadcast side of your network was still sending hello packets to the address, which the NBMA side was listening to. The NBMA side will respond to those hello packets with unicast hellos, forming the adjacency. Indeed, as you can see from this NetworkLessons note, in a pure NBMA scenario, you only need the network command to be issued on one of the two neighbors, and the adjacency will still form.

However, this is not a reliable or recommended setup, as the NBMA side won’t ever be able to initiate hellos, and must rely on the other neighbor to initiate the formation of the adjacency.

I hope this has been helpful!