In a network where IP multicast traffic is transmitted for multimedia applications, traffic is blocked at routed interface (VLAN) boundaries unless a multicast routing protocol is running. PIM is a family of routing protocols that form multicast trees to forward traffic from multicast sources to subnets are using a protocol such as IGMP to request the traffic. PIM relies on the unicast routing tables created by any of several unicast routing protocols to identify the path back to a multicast source, known as reverse path forwarding (RPF.) Based on information provided by the unicast routing tables, PIM sets up a distribution tree for the multicast traffic. The PIM-DM and PIM-SM protocols on the switches enable and control multicast traffic routing.

IGMP provides the multicast traffic link between a host and a multicast router running PIM-DM or PIM-SM. IGMP and either PIM-DM or PIM-SM must be enabled on VLANs whose member ports have directly connected hosts with a valid need to join multicast groups. PIM-DM is used in networks where, at any given time, multicast group members exist in relatively large numbers and are present in most subnets.