User manual

Publication 1783-UM003D-EN-E - December 2009 37
Chapter 2
Storm control (or traffic suppression) monitors packets passing from an
interface to the switching bus and determines if the packet is unicast, multicast,
or broadcast. The switch counts the number of packets of a specified type
received within the 1-second time interval and compares the measurement
with a predefined suppression-level threshold.
Storm control uses one of these methods to measure traffic activity:
• Bandwidth as a percentage of the total available bandwidth of the port
that can be used by the broadcast, multicast, or unicast traffic.
• Traffic rate in packets per second at which broadcast, multicast, or
unicast packets are received.
• Traffic rate in bits per second at which broadcast, multicast, or unicast
packets are received.
With each method, the port blocks traffic when the rising threshold is reached.
The port remains blocked until the traffic rate drops below the falling
threshold and then resumes normal forwarding. In general, the higher the
level, the less effective the protection against broadcast storms.
The graph shows broadcast traffic patterns on an interface over a given period
of time. The example can also be applied to multicast and unicast traffic. In
this example, the broadcast traffic being forwarded exceeded the configured
threshold between time intervals T1 and T2 and between T4 and T5. When
the amount of specified traffic exceeds the threshold, all traffic of that kind is
dropped for the next time period. Therefore, broadcast traffic is blocked
during the intervals following T2 and T5. At the next time interval (for
example, T3), if broadcast traffic does not exceed the threshold, it is again
forwarded.
IMPORTANT
When the storm control threshold for multicast traffic is
reached, all multicast traffic except network management
traffic, such as bridge protocol data unit (BDPU) and Cisco
Discovery Protocol (CDP) frames, are blocked.