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Multicasting and Non-Broadcast Multi-Access Networks

Multicasting and Non-Broadcast Multi-Access Networks

A non-broadcast multi-access (NBMA) network, such as frame relay, needs special consideration in regards to multicast traffic. The network in Figure 9-10 is a partially meshed frame relay network configured as a hub and spoke arrangement.

Figure 9-10: Partially meshed Non-Broadcast Multi-Access (NBMA) network

If the hub router needs to send a broadcast to every spoke router, then the broadcast packet needs to be replicated and sent four times, once to each spoke router. This is not a problem with an occasional broadcast packet, yet with multicast traffic this method of operation can dramatically affect the bandwidth utilization on the frame relay network. For example, assume the hub router receives multicast traffic for groups that only router B and C have joined. The multicast traffic would be replicated and sent to routers A, B, C, and D, even though A and D do not have receivers. We also assume here that all four spoke routers are running PIM. To override this behavior, configure the interface in NBMA mode.

interface serial 0

ip pim nbma-mode

ip pim sparse-mode

When the hub router receives a Join from one of the spoke routers, the router records the group and the address of the joiner. Therefore, when the hub router receives a multicast packet to be forwarded over the frame relay network, the packet is only sent to the spoke routers that have joined the group. When a spoke router sends a Prune to leave the group, the forwarding entry is then deleted on the hub router. This command only works with PIM-Sparse Mode.