Protocol Independent Multicast - Sparse Mode (PIM-SM) overview

In a network, IP multicast traffic transmitted for multimedia applications is blocked at routed interface boundaries unless a multicast routing protocol is running. Protocol Independent Multicast (PIM) is a family of routing protocols. It forms multicast trees to forward traffic from multicast sources to subnets which use protocols such as IGMP and MLD to request the traffic.

PIM relies on the unicast routing tables to identify the path back to a multicast source (reverse path forwarding (RPF)). The unicast routing protocols create the unicast routing tables. With this information, PIM sets up the distribution tree for the multicast traffic.

PIM-Sparse Mode (PIM-SM) can be configured on physical ports, VLAN interfaces, LAG interfaces, and loopback interfaces. All such configurations work in the mentioned interfaces context.

IGMP/MLD provides the multicast traffic link between a host and a multicast router running PIM-SM. Both PIM-SM and IGMP/MLD must be enabled on VLANs whose member ports have directly connected hosts with a valid need to join multicast groups.

PIM-SM uses the pull mode for multicast forwarding, and it is suitable for large and medium-sized networks with sparsely and widely distributed multicast group members.

PIM-SM assumes that most hosts do not want to receive multicast traffic. It uses a nonflooding multicast model to direct traffic from the source to the interface when there are multicast receivers in the group. As a result, this model sends traffic only to the routers that specifically request it.