State-refresh packets and bandwidth conservation

A multicast switch, if directly connected to a multicast source (such as a video conference application), periodically transmit 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. This means that 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.


Some vendors' multicast routers do not offer the state-refresh feature. In this case, PIM-DM must periodically advertise an active multicast group to these devices by repeating the flood/prune cycle on the paths to such routers. For better traffic management in multicast-intensive networks where some multicast routers do not offer the state-refresh feature, you may want to group such routers where the increased bandwidth usage will have the least effect on overall network performance.

Bandwidth conservation in switches with PIM-DM state-refresh