Some designs cause unnecessary traffic flow on links in a PIM-SM domain. In these cases, traffic is not duplicated to the receivers, but wastes bandwidth.
The following figure shows such a situation. Switch B is the DR between switches A and B. Switch C is the RP. A receiver R is on the VLAN (V1) that connects switches A and B. A source sends multicast data to the receiver.
IGMP reports that the messages that the receiver sends are forwarded to the DR, and both A and B create (*,G) records. Switch A receives duplicate data through the path from C to A, and through the second path from C to B to A. Switch A discards the data on the second path (assuming the upstream source is A to C).
To avoid this waste of resources, do not place receivers on V1. This configuration guarantees that no traffic flows between B and A for receivers attached to A. In this case, the existence of the receivers is only learned through PIM join messages to the RP [for (*,G)] and of the source through SPT joins.