Multicast tunnels provide support for
multicasting across networks that are separated by unicast routers that do not support IP multicasting.A
tunnel is a virtual point-to-point link between a pair of multicast routers located anywhere in an internet. The multicast router encapsulates IP multicast packets for transmission through tunnels so that they appear as unicast datagrams to intervening routers and subnetworks.The
multicast router adds the encapsulation when a packet enters a tunnel, and removes it when the packet exits from a tunnel. The packets are encapsulated using the IP-in-IP protocol (IP protocol number 4).The tunneling mechanism establishes a
virtual internet, for the purpose of multicasting only, which is independent of the physical internet, and which can span multiple Autonomous Systems.Multicast tunnels are used by
Distance-Vector Multicast Routing Protocol (DVMRP).