This counter reports the total bytes of Layer-2 multicast traffic on this port that exceeded the configured flood/storm-control threshold and were therefore dropped rather than forwarded, during the current, still-filling 15-minute window. A nonzero or rising value building up right now in the still-accumulating current interval is the storm-control mechanism actively protecting the rest of the network from a Layer-2 multicast flood, so it directly signals an unregistered multicast stream that no IGMP membership ever pruned to specific ports is happening on this specific port, not a general congestion issue elsewhere in the network. For example, an unregistered multicast stream that no IGMP membership ever pruned to specific ports could push this counter from near zero up to several megabytes within the still-accumulating current interval before dropping back to zero once the source is disconnected -- exactly the transient event this counter exists to catch.