What is NBMA?

NBMA (Non-Broadcast Multi-Access) describes a network where many routers share one medium and can each reach the others, but broadcast and multicast don't work.

The classic examples are Frame Relay, ATM and X.25, where a provider built virtual circuits between sites. Several routers sat on one subnet, so the network was multi-access like an Ethernet LAN, but a router couldn't send one packet and have it reach everyone. That breaks protocols that discover neighbours by multicast. OSPF normally sends hellos to 224.0.0.5, so on its NBMA network type you configure each neighbour's address by hand and the router sends unicast hellos to each one. OSPF still elects a DR and BDR on NBMA, and with longer default timers than on broadcast networks: a 30-second hello and 120-second dead interval.

The DR election is where NBMA designs usually go wrong. In a hub-and-spoke network, only the hub has circuits to every spoke, so the hub must become the DR; spokes are given priority 0 so they can never win. If a spoke wins, other spokes can't reach it and LSAs don't flood properly. Because this is fiddly, many engineers used OSPF's point-to-multipoint network type on such links instead, which needs no DR and treats each circuit as its own link. Frame Relay and ATM have largely gone, but the idea survives wherever multicast is missing, such as some cloud networks and tunnel overlays (DMVPN is the common example). If OSPF hellos never arrive on a network that can only pass unicast, the network type is the first thing to check. The OSPF Fundamentals lesson explains the DR election on broadcast networks.