There are several problems when it comes to multicast on NBMA networks. First PIM treats the interfaces connected to NBMA networks as broadcast capable, even though they are not. So Assert messages are sent out to elect the forwarder for the “broadcast” network. Second, due to the split-horizon rule, the hub will not forward multicast packets it receives on its hub interface to other spokes on that interface.
The solution is to inform PIM of the true Layer 2 topology (non-broadcast). PIM NBMA is a special extension to PIM protocol operation that treats the underlying Layer 2 network as a collection of Point-to-point circuits. PIM NBMA is only supported in sparse mode.
R1(config)# ip multicast routing distributed
R1(config)# ip pim sparse-mode
R1(config)# ip pim nbma-mode
R1(config)# exit