What is a Shared Tree (ST), and a Shortest Path Tree (SPT) ?
The Shared Tree (also known as the Core Based Tree, or CBT, from the different routing protocol described in RFC 2189) is the set of paths from the RP to the complete set of group members. The Shortest Path Tree is the similar set of paths from a group source to the complete set of group members. It is called “shortest path” as the unicast routing table is used to determine the best route between the receiver and the source, which is assumed to be the best route between the source and the receiver. It may not be the “shortest” route in a geographical sense. Note also that the Shared Tree really is the Shortest Path Tree to the RP; but it will be generally suboptimal for transmissions from an arbitrarily located source. Having a long lasting Shared Tree could be very useful for some multicast applications, such as video-conferencing, where sources might join the group for brief intervals, and then leave the group. In this case, it might not be worth setting up a separate SPT for each tr
