A traffic storm is generated when messages are broadcast on a network and each message prompts a receiving node to respond by broadcasting its own messages on the network. This, in turn, prompts further responses, creating a snowball effect. The LAN is suddenly flooded with packets, creating unnecessary traffic that leads to poor network performance or even a complete loss of network service. Storm control enables the switch to monitor traffic levels and to drop broadcast, multicast, and unknown unicast packets when a specified traffic level—called the storm control level—is exceeded, thus preventing packets from proliferating and degrading the LAN. As an alternative to having the switch drop packets, you can configure it to shut down interfaces or temporarily disable interfaces (see the action-shutdown statement or the port-error-disable statement) when the storm control level is exceeded.

The default configuration of storm control differs according to the switch line:

• On EX2200, EX3200, EX4200 switches and on EX6200 access ports—Storm control does not apply to multicast traffic by default. The factory default configuration enables storm control for broadcast and unknown unicast traffic on all switch interfaces, with the storm control level set to 80 percent of the combined broadcast and unknown unicast streams.
• On EX4500 and EX8200 switches—The factory default configuration enables storm control on all switch interfaces, with the storm control level set to 80 percent of the combined broadcast, multicast, and unknown unicast streams.

You can customize the configuration of storm control, as follows:

• You can change the storm control level for a specific interface by configuring the bandwidth value for the combined traffic streams that are subject to storm control on that interface. The type of traffic stream (broadcast, unknown unicast, and multicast) that is included within the bandwidth consideration depends on which types of traffic are enabled for storm control monitoring on that interface.
• You can enable storm control selectively for multicast traffic on a specific interface or on all interfaces.

  Note: We do not recommend enabling storm control for multicast traffic on aggregated Ethernet interfaces on EX2200, EX3200, EX3300, EX4200, and EX6200 switches.

• On all switches—You can disable storm control selectively for either broadcast streams, or multicast streams, or for unknown unicast streams.
• On EX8200 switches—You can also disable storm control selectively for either registered multicast traffic, or unregistered multicast traffic, or for both types of multicast traffic.

The sending and receiving of broadcast, multicast, and unicast packets are part of normal LAN operation, so to recognize a storm, you must be able to identify when traffic has reached a level that is abnormal for your LAN. Suspect a storm when operations begin timing out and network response times slow down. As more packets flood the LAN, network users might be unable to access servers or e-mail.

Monitor the level of broadcast, multicast, and unknown unicast traffic in the LAN when it is operating normally. Use this data as a benchmark to determine when traffic levels are too high. Then configure storm control to set the level at which you want to drop broadcast traffic, multicast traffic, unknown unicast traffic, or two or all three of those traffic types.

Note: When you configure storm control bandwidth on an aggregated Ethernet interface, the storm control level for each member of the aggregated Ethernet interface is set to that bandwidth. For example, if you configure a storm control bandwidth of 15,000 Kbps on ae1, and ae1 has two members, ge-0/0/0 and ge-0/0/1, each member has a storm control level of 15,000 Kbps. Thus, the storm control level on ae1 allows a traffic rate of up to 30,000 Kbps of combined traffic streams. Traffic might include broadcast, multicast, and unknown unicast traffic, depending upon the configuration.