PIM-SM overview
In a network where IP multicast traffic is transmitted for multimedia applications, such traffic is blocked at routed interface (VLAN) boundaries unless a multicast routing protocol is running. Protocol Independent Multicast (PIM) is a family of routing protocols that form multicast trees to forward traffic from multicast sources to subnets that have used 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 (reverse path forwarding, or RPF.) With this information, PIM sets up the distribution tree for the multicast traffic. The PIM-DM and PIM-SM protocols on the switches covered in this guide enable and control multicast traffic routing.
IGMP provides the multicast traffic link between a host and a multicast router running PIM-SM. Both PIM-SM and IGMP 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. However, using PIM-DM in networks where multicast sources and group members are sparsely distributed over a wide area can result in unnecessary multicast traffic on routers outside the distribution paths needed for traffic between a given multicast source and the hosts belonging to the multicast group. In such networks, PIM-SM can be used to reduce the effect of multicast traffic flows in network areas where they are not needed. And because PIM-SM does not automatically flood traffic, it is a logical choice in lower bandwidth situations.