Storm Control

Rate limiting of layer 2 traffic is needed in port level of a switch to overcome some of the worst nightmare attacks in networking. Storms can be unicast, multicast or broadcast..

Controlling storms can be done by setting rising & falling thresholds based on followings..

1) Packet rate
2) Percentage of the interface bandwidth


When any of the configured threshold is passed, the switch can take following actions..

1) Discarding excess traffic according to the configured commands
2) Shut down the port or send an SNMP trap

Let's see a real world configuration requirement..

(1) Limit broadcast traffic to 100 packets per second. When broadcast traffic drops back to 50 packets per second, begin forwarding broadcast traffic again.
(2) Limit multicast traffic to 0.5% of the interface. When multicast traffic drops to 0.4% begin forwarding multicast traffic again.
(3) Limit unicast traffic to 80% of the interface, forward all unicast traffic is up to this limit.
(4) Send an SNMP trap for above conditions.

Following show commands will confirm the configurations..

As you can see in the 3rd task, if you haven't configured a falling threshold switch will not wait to forward traffic until a lower threshold.

Note:- 

Storm control can be done in physical ports only. Though the commands are visible in etherchannel interfaces they don't work..

Here is a good post I found online explaining how the traffic limiting is done with poling intervals
http://packetlife.net/blog/2008/nov/27/storm-control/