Why Rendezvous Points?
A Rendezvous Point (RP) is used as a temporary way to connect a would-be multicast receiver to an existing shared multicast tree passing through the rendezvous point. When volume of traffic crosses a threshold, the receiver is joined to a source-specific tree, and the feed through the RP is dropped. You can think of this as obtaining copies of something through a friend who already subscribes, and when it proves useful or interesting, it’s worth the bother to become a direct subscriber. So: the scalable way to do multicast is PIM Sparse Mode (PIM-SM), and PIM-SM requires that you have at least one RP.
