Cisco WS-C3020 Software Configuration Manual page 492

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
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broadcast, multicast, or unicast traffic
Traffic rate in packets per second at which broadcast, multicast, or unicast packets are received
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Traffic rate in bits per second at which broadcast, multicast, or unicast packets are received
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Traffic rate in packets per second and for small frames. This feature is enabled globally. The
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threshold for small frames is configured for each interface. (Cisco IOS Release 12.2(44)SE or later)
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.
When the storm control threshold for multicast traffic is reached, all multicast traffic except control
Note
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 and regular multicast data
traffic, so both types of traffic are blocked.
The graph in
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.
Figure 23-1
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.
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
23-2
Figure 23-1 shows broadcast traffic patterns on an interface over a given period of time.
Broadcast Storm Control Example
0 T1 T2 T3
Chapter 23
Threshold
T4 T5 Time
Configuring Port-Based Traffic Control
Forwarded traffic
Blocked traffic
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