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Configuring Storm Control
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 (if one is specified) and then resumes normal
forwarding. If the falling suppression level is not specified, the switch blocks all traffic until the traffic
rate drops below the rising suppression level. In general, the higher the level, the less effective the
protection against broadcast storms.
Note
When the storm control threshold for multicast traffic is reached, all multicast traffic except control
traffic, such as bridge protocol data unit (BDPU) and Cisco Discovery Protocol (CDP) frames, are
blocked. However, the switch does not differentiate between routing updates, such as OSPF, and regular
multicast data traffic, so both types of traffic are blocked.
The graph in
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.
Figure 21-1
Total
number of
broadcast
packets
or bytes
The combination of the storm-control suppression level and the 1-second time interval controls the way
the storm control algorithm works. A higher threshold allows more packets to pass through. A threshold
value of 100 percent means that no limit is placed on the traffic. A value of 0.0 means that all broadcast,
multicast, or unicast traffic on that port is blocked.
Note
Because packets do not arrive at uniform intervals, the 1-second time interval during which traffic
activity is measured can affect the behavior of storm control.
You use the storm-control interface configuration commands to set the threshold value for each traffic
type.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
21-2
Figure 21-1
shows broadcast traffic patterns on an interface over a given period of time. The
Broadcast Storm Control Example
0
T1
T2
T3
Chapter 21
Threshold
T4
T5
Time
Configuring Port-Based Traffic Control
Forwarded traffic
Blocked traffic
78-17058-01
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