When a broadcast storm occurs, even with STP enabled, there is another option to protect your network from broadcast flooding. This method is called storm-control. The technics of this protocol are simple:
Stom control configuration is based on a per-port bases and storm control can be enabled for broadcasts, unicasts and/or multicasts. When configuring stormcontrol, you have to specify the rising and falling values: the rising value is the value on which an action should occur (rate-limit the traffic to this value, err-disable this port or send a SNMP trap). The rising value is the value on which the action will be undone.