This paper first explains why IP multicasting must be used. Then it summaries the fundamentals of IP multicasting and multicast routing protocols. After that it specially discusses protoool independent multicast-sparse mode (PIM-SM), points out its defect and proposes a scheme to solve it. In this scheme each source registers and sends data packets toward multiple rendezvous points (RPs) but receivers only join to a single RP. If one of the RPs fails, receivers will join to one of the alternative RPs quickly. On implementation, the paper centers on the details of implementing PIM-SM. Finally it discusses the problems and prospect of IP multicasting.