Cisco 3845 - Security Bundle Router Software Manual page 372

Configuring Storm Control
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, such as OSPF, and regular
multicast data traffic, so both types of traffic are blocked.
When storm control is enabled, the switch monitors packets entering an interface and determines if the
packet is unicast, multicast, or broadcast. The switch monitors the number of broadcast, multicast, or
unicast packets received within a 200-millisecond 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
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 19-1
Total
number of
broadcast
packets
or bytes
The combination of the storm-control suppression level and the 200 ms 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.
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.
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.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
19-2
Figure 19-1 shows broadcast traffic patterns on an interface over a given period of time.
Broadcast Storm Control Example
0 T1 T2 T3
Chapter 19
Threshold
T4 T5 Time
Configuring Traffic Control
Forwarded traffic
Blocked traffic
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