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Multicast over GRE
On a network, multicast data sent from a multicast source is transmitted by routers hop by hop. If a next-hop router does not support multicast routing, the source and receivers are separated by a unicast network. A Generic Routing Encapsulation (GRE) tunnel can be set up between multicast routers at both ends of the unicast network to transmit multicast data across the unicast network.
As shown in Figure 7-10, Router1 and Router6 are multicast routers, whereas Router2 to Router5 are unicast routers that do not support multicast routing. On this network, a GRE tunnel can be set up between Router1
and Router6 to transmit multicast data. Router1 encapsulates multicast data packets in unicast IP packets and sends the IP packets to routers on the unicast network. The unicast routers forward the IP packets to Router6. After receiving the IP packets,
Router6 removes IP headers from the packets and sends multicast packets to the receiver.
