The PIM-DM State Refresh feature keeps the PIM-DM prune state from timing out by periodically sending a state refresh control message down the source tree. These control messages reset the prune hold time timer on each pruned interface and prevent the bandwidth waste that occurs with each prune, time-out, and flood cycle.
When a topology change occurs, the PIM-DM State Refresh feature improves network convergence. For example, suppose that an S, G entry on S5 in the following figure is removed due to non-availability of a route. Without PIM-DM State Refresh, multicast traffic is blocked for minutes (due to a time-out on the upstream routers). In the meantime if an IGMP (Internet Group Management Protocol) member or a PIM-DM neighbor joins S5, there is no way to pull traffic down immediately because S5 does not have any S, G information. State refresh control messages solve this problem by indicating S, G state information periodically to all downstream routers. When S5 receives a state refresh from S3, it scans the S, G information and all pending requests from PIM-DM neighbors and IGMP members. If there are pending requests for the group in the state refresh message, S5 can immediately send a graft message upstream to circumvent the upstream timers and pull multicast traffic to its members and neighbors.
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