Cisco WS-C3550-12G Software Configuration Manual page 342

Configuring Storm Control
Note When the rate of multicast traffic exceeds a set threshold, all incoming traffic (broadcast, multicast,
and unicast) is dropped until the level drops below the threshold level. Only spanning-tree packets
are forwarded. When broadcast and unicast thresholds are exceeded, traffic is blocked for only the
type of traffic that exceeded the threshold.
When storm control is enabled, the switch monitors packets passing from an interface to the switching
bus and determines if the packet is unicast, multicast, or broadcast. The switch monitors the number of
broadcast, multicast, or unicast packets received within the 1-second time interval, and when a threshold
for one type of traffic is reached, that type of traffic is dropped. This threshold is specified as a
percentage of total available bandwidth that can be used by broadcast (multicast or unicast) traffic.
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 12-1 Broadcast Storm Control Example
Total
number of
broadcast
packets
or bytes
The combination of the storm-control suppression level and the 1-second time interval control 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.
Because packets do not arrive at uniform intervals, the 1-second time interval during which traffic
Note
activity is measured can affect the behavior of storm control.
The switch continues to monitor traffic on the port, and when the utilization level is below the threshold
level, the type of traffic that was dropped is forwarded again.
You use the storm-control interface configuration commands to set the threshold value for each traffic
type.
Catalyst 3550 Multilayer Switch Software Configuration Guide
12-2
Figure 12-1 shows broadcast traffic patterns on an interface over a given period of time. The
0 T1 T2 T3
Chapter 12
Threshold
T4 T5 Time
Configuring Port-Based Traffic Control
Forwarded traffic
Blocked traffic
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