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
.