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Leaf and Branch

Each subnetwork with hosts on the router that has at least one interested receiver is a leaf on the distribution tree. Routers can have multiple leaves on different interfaces and must send a copy of the IP multicast packet out on each interface with a leaf. When a new leaf subnetwork is added to the tree (that is, the interface to the host subnetwork previously received no copies of the multicast packets), a new branch is built, the leaf is joined to the tree, and replicated packets are now sent out on the interface. The number of leaves on a particular interface does not affect the router. The action is the same for one leaf or a hundred.

When a branch contains no leaves because there are no interested hosts on the router interface leading to that IP subnetwork, the branch is pruned from the distribution tree, and no multicast packets are sent out that interface. Packets are replicated and sent out multiple interfaces only where the distribution tree branches at a router, and no link ever carries a duplicate flow of packets.

Collections of hosts all receiving the same stream of IP packets, usually from the same multicast source, are called groups. In IP multicast networks, traffic is delivered to multicast groups based on an IP multicast address, or group address. The groups determine the location of the leaves, and the leaves determine the branches on the multicast network.

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