Protocol Independent Multicast - Dense Mode (PIM-DM) 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. This routing method is known as 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-DM operates at the router level to direct traffic for a particular multicast group along the most efficient path to the network which has hosts that have joined that group. A unicast source address and a multicast group address comprise a given source/group (S/G) pair. Multicast traffic moving from a source to a multicast group address creates a flow to one or more areas of the network requiring the traffic. The flow destination is the multicast group address and not a specific host or VLAN. A single multicast flow has one source and one multicast group address (destination), but may reach many hosts in different subnets, depending on which hosts have issued joins for the same multicast group.

PIM routes the multicast traffic for a particular S/G pair on paths between the source unicast address and to the interfaces where it is requested (by joins from hosts connected to those subnets.) Physical destinations for a particular multicast group can be hosts in different networks. Individual hosts use IGMP/MLD configured per-subnet to send joins requesting membership in a particular multicast group. All hosts that have joined a given multicast group (defined by a multicast address) remain in that group as long as they continue to issue periodic joins.

PIM-DM interoperates with IGMP/MLD and the switch's routing protocols. PIM operates independently of the routing protocol that is chosen to run on the switches. So PIM-DM can be used with RIP, OSPF, BGP, or static routes configured. PIM-DM uses a unicast routing table to find the path to the originator of the multicast traffic and sets up multicast trees for distributing multicast traffic.