9.3 Routing Multicast Traffic
9.3.3
Source distribution tree
The simplest form of a multicast distribution tree is a source tree.  Its root is the source of the multicast traffic and whose branches form a Spanning Tree through the network to the receivers. Because this tree uses the shortest path through the network, it also is referred to frequently as a shortest-path tree (SPT).

Figure shows an example of an SPT for Group 224.1.1.1 rooted at the source, Host A, and connecting two receivers, Hosts B and C.

The special notation of (S,G), pronounced "S comma G," enumerates an SPT where S is the IP address of the source and G is the multicast group address. Using this special notation, the SPT for the example in Figure would be written as (192.1.1.1, 224.1.1.1).

Notice that this notation implies that a separate SPT exists for every individual source originating a stream for a mulitcast group. Therefore, if Host B is also sending traffic to Group 224.1.1.1 and Hosts A and C are receivers, then a separate (S,G) SPT would exist with a notation of (192.2.2.2, 224.1.1.1), as shown in Figure .