PIM

Protocol Independent Multicast. A set of protocols that are used by multicast IP routers to build a map with a tree structure, with branches going to all the receiving computers from roots at the sources of multicast streams.

There are several flavors of PIM. Allows the routers to know where to send Multicast traffic. It sits on top of standard routing protocols like OSPF, which are still needed when using PIM (unless static routing is being used)

PIM-SM: Sparse Mode – Builds shared tee from a rooted at a Rendezvous Point (which is often, but not always manually configured). Branches of the tree will only be extended to routers on which there are devices that require multicast streams.

PIM-SSM: Source Specific Mode: Similar to PIM-SM about allows trees to be built that are routed from just one source device.

PIM-DM: Dense Mode. Send streams to all hosts and then prunes back paths to hosts that don’t want to receive. Easy to set up, but wasteful unless most devices want to receive most streams most of the time.

Less commonly used alternatives to PIM are DVMRP and Multicast OSPF

See Multicast, IGMP, Router