Protocol Independent Multicast Overview
Protocol Independent Multicast (PIM) is used for efficient routing to multicast groups that might span wide-area and interdomain internetworks. It is called “protocol independent” because it does not depend on a particular unicast routing protocol. The Junos OS supports sparse mode, dense mode, and sparse-dense mode.
For information about standards supported for PIM, see IP Multicast Specifications.
Because the PIM mode you choose determines the PIM configuration properties, you first must decide whether PIM operates in sparse, dense, or sparse-dense mode in your network. Each mode has distinct operating advantages in different network environments.
In sparse mode, routers must join and leave multicast groups explicitly. Upstream routers do not forward multicast traffic to a router unless it has sent an explicit request (by means of a join message) to the rendezvous point (RP) router to receive this traffic. The RP serves as the root of the shared multicast delivery tree and is responsible for forwarding multicast data from different sources to the receivers.
Sparse mode is well suited to the Internet, where frequent interdomain joins and prunes are common.
Unlike sparse mode, in which data is forwarded only to routers sending an explicit PIM join request, dense mode implements a flood-and-prune mechanism, similar to the Distance Vector Multicast Routing Protocol (DVMRP). In dense mode, a router receives the multicast data on the incoming interface, then forwards the traffic to the outgoing interface list. Flooding occurs periodically, and is used to refresh state information, such as the source IP address and multicast group pair. If the router has no interested receivers for the data, and the outgoing interface list becomes empty, the router sends a PIM prune message upstream.
Dense mode works best in networks where few or no prunes occur. In such instances, dense mode is actually more efficient than sparse mode.
Sparse-dense mode, as the name implies, allows the interface to operate on a per-group basis in either sparse or dense mode. A group specified as “dense” is not mapped to an RP. Instead, data packets destined for that group are forwarded by means of PIM dense mode rules. A group specified as “sparse” is mapped to an RP, and data packets are forwarded by means of PIM sparse-mode rules. Sparse-dense mode is useful in networks implementing auto-RP for PIM sparse mode.
