PIM

Protocol-Independent Multicast (PIM) can forward multicast packets with a unicast protocol. PIM efficiently routes multicast traffic for groups that span wide area (and inter-domain) networks. It works with all existing unicast routing protocols. PIM supports three modes: dense, sparse and Source-Specific Multicast (SSM). You can enable only one mode of PIM at a time.

Note - Due to an OS limitation, PIM supports only 31 interfaces. If more are configured, PIM runs only on the first 31 interfaces.

Dense Mode

Dense mode is most useful when:

• Senders and receivers are in close proximity to one another.
• There are few senders and many receivers.
• The volume of multicast traffic is high.
• The stream of multicast traffic is constant.

Sparse Mode

Sparse mode is most useful when:

• There are few receivers in a group.
• Senders and receivers are separated by WAN links.
• The type of traffic is intermittent.

Source-Specific Multicast (SSM) Mode

Source-Specific Multicast (SSM) is most useful when:

• Most multicast traffic is from well-known sources.
• It is desirable to avoid the overhead of shared tree and rendezvous point processing associated with sparse mode.

SSM is a version of PIM sparse-mode. It is used in conjunction with IGMP version 3 to request or block multicast traffic from specific sources. For example, when a host requests traffic for a multicast group from a specific source, SSM sends PIM join/prune messages towards the source.

The multicast group range 232/8 is reserved for SSM. When SSM is enabled, sparse-mode accepts only IGMPv3 reports for groups that fall within this range. Sparse-mode ignores IGMP v1 and v2 reports in this range. In addition, only shortest-path-tree (SPT) join/prune messages for these groups are accepted from neighboring routers. All other multicast groups are processed as in native sparse mode.

SSM does not need a rendezvous-point (RP). The presence of an RP for any of the SSM groups does not have any influence on the processing of join/prune messages.

Dense Mode State Refresh

The PIM Dense Mode State Refresh option can be used in conjunction with dense mode to eliminate the periodic flood-and-prune of multicast data with no active receivers. All PIM routers must have State Refresh enabled to take advantage of this feature.

PIM Dense Mode builds multicast distribution trees that operate on a flood and prune principle. Multicast packets from a source are flooded throughout a PIM dense mode network. PIM routers that receive multicast packets and have no directly connected multicast group members or PIM neighbors send a prune message back up the source-based distribution tree toward the source of the packets. As a result, subsequent multicast packets are not flooded to pruned branches of the distribution tree. However, the pruned state in PIM dense mode times out approximately every three minutes and the entire PIM dense mode network is reflooded with multicast packets and prune messages. This reflooding of unwanted traffic throughout the PIM dense mode network consumes network bandwidth unnecessarily.

Use the PIM Dense Mode State Refresh feature to keep the pruned state in PIM dense mode from timing out by periodically forwarding a control message down the distribution tree. The control message refreshes the prune state on the outgoing interfaces of each router in the tree. This saves network bandwidth by greatly reducing the reflooding of unwanted multicast traffic to pruned branches of the PIM dense mode network.