PIM-DM features

Multicast flow management

Multicast flow management refers to how the routing switch manages forwarding and pruned flows. This is useful when planning topologies to include multicast support and when viewing and interpreting the show command output for PIM-DM features.

Initial flood and prune

When a router running PIM-DM receives a new multicast flow, it initially floods the traffic to all downstream multicast routers. Branches that do not have members send Prune messages toward the source to prune off the unwanted/unnecessary traffic.

Maintaining the prune state

For a multicast group "X" on a given interface, when the last host belonging to group "X" leaves the group, PIM places that interface in a prune state. Multicast traffic from group "X" is now blocked to that interface. The prune state remains until a host on the same interface issues a join for group "X", in which case the router cancels the prune state and changes the flow to the forwarding state.

State-refresh packets and bandwidth conservation

A multicast switch, if directly connected to a multicast source (such as a video conference application), periodically transmits state-refresh packets to downstream multicast routers. On routers that have pruned the multicast flow, the state-refresh packets keep the pruned state alive. On routers that have been added to the network after the initial flooding and pruning of a multicast group, the state-refresh packets inform the newly added router of the current state of that branch. So if all multicast routers in a network support the state-refresh packet, the multicast router directly connected to the multicast source performs only one flood-prune cycle to the edge of the network when a new flow (multicast group) is introduced and preserves bandwidth for other uses.