PIM Overview

Protocol Independent Multicast (PIM) is the de-facto standard for routing multicast traffic over the Internet. Other multicast routing protocols such as DVMRP and MOSPF are sometimes used in controlled environment, but are not widely deployed. PIM does not depend on a particular unicast routing protocol for its operation. Also, it does not have any mechanism of its own for route discovery. PIM operation is based on the routing table being populated by another routing protocol, or by the user. This provides flexibility in routing unicast and multicast traffic based on a common database.

PIM has two flavors, sparse and dense mode, that are deployed in different topologies. These two flavors, called PIM-SM and PIM-DM, are different in operation. PIM-SM is based on a "join protocol", where traffic is not forwarded on a segment unless an explicit request originates (typically through IGMP (Internet Group Management Protocol)) from the network segment. PIM-DM is based on a "flood and prune" mechanism, where every one receives traffic until they explicitly inform (through the PIM-DM prune mechanism) that they do not want that particular stream. Thus, PIM-DM is typically deployed in topologies where listeners are densely populated. And PIM-SM is typically deployed where the receivers are sparsely populated over the network, so that most of the network segments‘ bandwidth is conserved.

You can configure dense mode or sparse mode on a per-interface basis. After they are enabled, some interfaces can run dense mode, while others run sparse mode. The switch supports both dense mode and sparse mode operation.