PIM Sparse Mode

These are the major characteristics of PIM sparse mode:

Routers with downstream receivers join a PIM sparse-mode tree through an explicit join message.
PIM sparse-mode RPs are the routers where receivers meet sources.
Senders announce their existence to one or more RPs, and receivers query RPs to find multicast sessions.
Once receivers get content from sources through the RP, the last-hop router (the router closest to the receiver) can optionally remove the RP from the shared distribution tree (*,G) if the new source-based tree (S,G) is shorter. Receivers then get content directly from the source.
This transitional aspect of PIM sparse mode from shared to source-based tree is one of the major attractions of PIM. This feature prevents overloading the RP or surrounding core links.

There are related issues regarding source, RPs, and receivers when sparse mode multicast is used:

Sources must be able to send to all RPs.
RPs must all know each other.
Receivers must send explicit join messages to a known RP.
Receivers initially need to know only one RP (they later learn about others).
Receivers can explicitly prune themselves from a tree.
Receivers that never transition to a source-based tree are effectively running CBT.

PIM sparse mode has standard features for all of these issues.