Overview

Note

PIM requires an advanced router license on the 7100-Series platform. See License Overview for 7100-Series advanced router license details.

PIM is made up of a collection of multicast routing protocols optimized for different multicast environments. The S- K- and 7100-Series platforms supports two PIM modes: sparse mode (SM) and dense mode (DM).

PIM dynamically builds a distribution tree for forwarding multicast data on a network. It is designed for use where there may be many devices communicating at the same time, and any one of the devices could be the sender at any particular time. Scenarios for using PIM multicasting include desktop video conferencing and telephone conference calls.

PIM relies on IGMP technology to determine group memberships and uses existing unicast routes to perform reverse path forwarding (RPF) checks, which are, essentially, a route lookup on the source. Its routing engine then returns the best interface, regardless of how the routing table is constructed. In this sense, PIM is independent of any routing protocol. It can perform RPF checks using protocol-specific routes (for example, OSPF routes), static routes, or a combination of route types.

Note

IGMP must be enabled for PIM to operate.

PIM-SM is a multicast routing protocol optimized for a network in which multicast recipients are sparsely distributed throughout the network. PIM-SM assumes that the network contains subnets that will not want a given multicast packet. Given this assumption, routers must explicitly tell their upstream neighbors about their interest in particular groups and sources. PIM-SM creates multicast distribution trees based upon the sending and receiving of PIM Join and Prune messages to join or leave the distribution tree.

PIM-SM by default uses shared trees, which are multicast distribution trees rooted at some selected node called the Rendezvous Point (RP) and used by all sources sending to the multicast group. PIM-SM is a soft-state protocol. All state periodically times out after receiving the control message that instantiated it. To keep the state alive, all PIM Join messages are periodically retransmitted.

PIM-DM is a multicast routing protocol optimized for a network in which the receivers for any multicast group are assumed to be densely distributed throughout the network. PIM-DM assumes that most, or at the very least, many subnets in the network will want any given multicast packet. PIM-DM uses source-based trees rather than the shared (RP-based) tree used by PIM-SM. PIM-DM does not use the concept of an RP in which all sources first send packets to a single router. PIM-DM initially sends multicast data to all hosts in the network. Routers that do not have any interested hosts then send PIM Prune messages to remove themselves from the tree. Because PIM-DM does not use RPs, it is easier to implement and deploy than PIM-SM. It is an efficient protocol when most receivers are interested in the multicast data, but does not scale well across larger domains in which most receivers are not interested in the data.