18.1.2 Notes on using storm control

(1) Action execution by storm detection and frame discard timing

The action to be performed when a storm is detected is performed when the specified receive rate exceeds the number of frames received within one second. On the other hand, to reduce the effect of burst reception in a short time, frames that exceed the acceptable reception rate are discarded at a shorter interval. Therefore, if a short burst of less than one second is received, the action may not be executed even if the acceptable receive rate is exceeded and the frame is discarded.

(2) About Executing Actions with Storm Detection

Actions to be performed when a storm is detected may take action at a receive rate lower than the specified receive rate, for example, when CPU is heavily loaded. Set the reception rate to a rate with sufficient room.

(3) Storm Detection and Recovery Detection

The Switch determines that a storm has occurred when the number of frames received per second exceeds the receive rate set in the configuration. After a storm occurs, if the number of frames received within one second remains at or below the receive rate for 30 seconds, it is judged that the storm has recovered.

If a port is blocked when a storm occurs, recovery from a storm cannot be detected because the port is no longer receiving any frames. If a port is shut down when a storm occurs, use a method other than the Switch, such as a network monitoring device, to confirm that the storm has recovered.

(4) Handling of unicast frames

For the Switch, unicast storm detection and the frames to be discarded are not the same. Unicast storm detection is performed on all unicast frames received on the port for which the acceptable receive rate is set. On the other hand, frames are discarded only for unicast frames that are flooded because the destination MAC address is not registered in MAC address table, and are counted when this number of frames exceeds the reception rate.

(5) Storm detection when using a filter

If filter discard and storm detection occur at the same time, more frames may be discarded, including frames that would otherwise be forwarded.

(6) Storm detection when bandwidth monitoring is used

If a bandwidth monitoring violation and storm detection occur at the same time, more frames may be discarded, including frames that would originally be forwarded.