Hi Josh,
The terms "broadcast networks" and "non-broadcast" networks (in short, BMA and NBMA, respectively) do not reflect on the type of traffic present in those networks, but rather at the capability of distinguishing and properly delivering multicasts and broadcasts in different network types.
Network technologies we're most familiar with, such as Ethernet or WiFi, are BMA type networks. The BMA network type is capable of delivering and replicating multicasts and broadcasts on its own. A sender simply sends a single multicast or a broadcast frame, and this frame will be multiplied (replicated) and delivered to all recipients by the network technology itself. The sender is not required to send multiple copies of the same multicast/broadcast frame, one for each intended recipient - quite the contrary, in BMA networks, the sender simply sends a single copy of every multicast/broadcast frame, and the network technology will take care itself to make sure that the frame gets copied to each recipient. Just to reiterate, Ethernet and WiFi technologies are prime examples of BMA type networks.
Not every network technology work this way, however. Technologies you're likely to learn about later, such as Frame Relay or Dynamic Multipoint VPN (DMVPN), lack this capability of automatically replicating a multicast/broadcast datagram and delivering it to proper recipients automatically. For example, a Frame Relay network operation is based on a notion of so-called virtual circuits - imaginary point-to-point links between devices (usually routers) attached to such a network. One router can have access to multiple virtual circuits, with each virtual circuit going to another single specific router. If the router in question has three virtual circuits over the single Frame Relay network, each of them going to a different neighboring router, and wants to send a broadcast that would be delivered to all those three neighboring routers, it would need to create three separate copies of the broadcast frame and send one of them over each virtual circuit. The Frame Relay network is incapable of replicating the broadcast frame itself because it has no idea that those three virtual circuits somehow belong together. As far as the Frame Relay network is concerned, each virtual circuit is entirely independent and unrelated to all other virtual circuits in the network, and frames carried over one virtual circuit can never, ever, leak into any other.
This is the point of NBMA-type networks. In their essence, NBMA networks make no difference between unicasts, multicasts and broadcasts. They deliver every type of traffic as unicast traffic. If a device connected to a NBMA network wants to send a multicast or broadcast frame that will be received by an entire group of other devices connected to the same NBMA network, it will need to replicate the multicast/broadcast traffic itself and send it in as many copies as how many recipients are there, one copy for each recipient.
So the fundamental difference between BMA and NBMA networks is:
- BMA networks will automatically replicate multicasts and broadcasts as they pass through them. A multicast/broadcast sender needs to send only a single copy of the multicast/broadcast flow. The BMA network will handle its replication and delivery to proper recipients. The sender neither knows nor cares about how many recipients are out there, or who they are.
- NBMA networks do not distinguish between unicasts, multicasts and broadcasts, and are not capable of replicating a single multicast/broadcast flow to multiple recipients. A multicast/broadcast sender has to know the exact identities of the intended recipients and has to send a separate copy of the multicast/broadcast flow to each of them individually.
Sometimes it is wrongly assumed that NBMA networks are incapable of carrying multicasts and broadcasts entirely. That is a misunderstanding. NBMA networks are perfectly capable of carrying multicast and broadcast traffic - they just don't treat it any differently than an ordinary unicast flow.
Would that answer your question? Feel welcome to ask further!
Best regards,
Peter
