Multicast Shortest-Path Tree
The distribution tree used for multicast is rooted at the source and is the shortest-path tree (SPT) as well. Consider a set of multicast routers without any active multicast traffic for a certain group (that is, they have no multicast forwarding state for that group). When a router learns that an interested receiver for that group is on one of its directly connected subnets, the router attempts to join the tree for that group.
To join the distribution tree, the router determines the unicast IP address of the source for that group. This address can be a simple static configuration on the router, or as complex as a set of protocols.
To build the SPT for that group, the router executes an a reverse path forwarding (RPF) check on the source address in its routing table. The RPF check produces the interface closest to the source, which is where multicast packets from this source for this group need to flow into the router.
The router next sends a join message out on this interface using the proper multicast protocol to inform the upstream router that it wants to join the distribution tree for that group. This message is an (S,G) join message because both S and G are known. The router receiving the (S,G) join message adds the interface on which the message was received to its output interface list (OIL) for the group and also performs an RPF check on the source address. The upstream router then sends an (S,G) join message out on the RPF interface toward the source, informing the upstream router that it also wants to join the group.
Each upstream router repeats this process, propagating joins out on the RPF interface, building the SPT as it goes. The process stops when the join message does one of two things:
Reaches the router directly connected to the host that is the source.
Reaches a router that already has multicast forwarding state for this source-group pair.
In either case, the branch is created, each of the routers has multicast forwarding state for the source-group pair, and packets can flow down the distribution tree from source to receiver. The RPF check at each router makes sure that the tree is an SPT.
SPTs are always the shortest path, but they are not necessarily short. That is, sources and receivers tend to be on the periphery of a router network, not on the backbone, and multicast distribution trees have a tendency to sprawl across almost every router in the network. Because multicast traffic can overwhelm a slow interface, and one packet can easily become a hundred or a thousand on the opposite side of the backbone, it makes sense to provide a shared tree as a distribution tree so that the multicast source can be located more centrally in the network, on the backbone. This sharing of distribution trees with roots in the core network is accomplished by a multicast rendezvous point. For more information about RPs, see Understanding Multicast Rendezvous Points, Shared Trees, and Rendezvous-Point Trees.