Understanding Storm Control - Cisco WS-C3750-48PS-S Software Configuration Manual

Configuring Storm Control

Understanding Storm Control

Storm control prevents switchports on a LAN from being disrupted by a broadcast, multicast, or unicast
storm on one of the physical interfaces. A LAN storm occurs when packets flood the LAN, creating
excessive traffic and degrading network performance. Errors in the protocol-stack implementation or in
the network configuration can cause a storm.
Storm control (or traffic suppression) monitors incoming traffic statistics over a time period and
compares the measurement with a predefined suppression level threshold. The threshold represents the
percentage of the total available bandwidth of the port. The switch supports separate storm control
thresholds for broadcast, multicast, and unicast traffic. If the threshold of a traffic type is reached, further
traffic of that type is suppressed until the incoming traffic falls below the threshold level.
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.
When storm control is enabled, the switch monitors packets passing from an interface to the switching
bus and decides if the packet is unicast, multicast, or broadcast. The switch monitors the number of
unicast, multicast, or broadcast 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 unicast, multicast, or broadcast 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 24-1 Broadcast Storm Control Example
Total
number of
broadcast
packets
or bytes
The combination of the storm-control suppression level and the 200-millisecond 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.
Catalyst 3750 Switch Software Configuration Guide
24-2
Figure 24-1 shows broadcast traffic patterns on an interface over a given period of time. The
0 T1 T2 T3
Chapter 24
Threshold
T4 T5 Time
Configuring Port-Based Traffic Control
Forwarded traffic
Blocked traffic
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