Multicast forwarding across unicast subnets
Devices forward the multicast data from a multicast source hop by hop along the forwarding tree, but some devices might not support multicast protocols in a network. When the multicast data is forwarded to a device that does not support IP multicast, the forwarding path is blocked. In this case, you can enable multicast forwarding across two unicast subnets by establishing a tunnel between the devices at the edges of the two unicast subnets.
Figure 30: Multicast data transmission through a tunnel
As shown in Figure 30, a tunnel is established between Device A and Device B. Device A encapsulates the multicast data in unicast IP packets, and forwards them to Device B across the tunnel through unicast devices. Then, Device B strips off the unicast IP header and continues to forward the multicast data to the receiver.
To use this tunnel only for multicast traffic, configure the tunnel as the outgoing interface only for multicast routes.