This counter reports the total bytes of unknown-unicast traffic on this port that exceeded the configured flood/storm-control threshold and were therefore dropped rather than forwarded, during a completed 15-minute window (for example 14:30-14:45). A nonzero or rising value accumulated over that completed 15-minute interval is the storm-control mechanism actively protecting the rest of the network from a unknown-unicast flood, so it directly signals a wave of frames destined for a MAC address that aged out of the forwarding table, or a MAC-flooding probe against the switch's CAM table is happening on this specific port, not a general congestion issue elsewhere in the network. For example, a wave of frames destined for a MAC address that aged out of the forwarding table, or a MAC-flooding probe against the switch's CAM table could push this counter from near zero up to several megabytes within that completed 15-minute interval before dropping back to zero once the source is disconnected -- exactly the transient event this counter exists to catch.