Catalyst 4500 Series Switch Software Configuration Guide, IOS XE 3.5.xE and IOS 15.2(1)Ex

Configuring Storm Control

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This chapter describes how to configure port-based traffic control on the Catalyst 4500 series switch.

This chapter consists of these sections:

• About Storm Control
• Enabling Broadcast Storm Control
• Enabling Multicast Storm Control
• Disabling Broadcast Storm Control
• Disabling Multicast Storm Control
• Displaying Storm Control

Note For complete syntax and usage information for the switch commands used in this chapter, see the Cisco Catalyst 4500 Series Switch Command Reference and related publications at this location:

http://www.cisco.com/en/US/products/hw/switches/ps4324/index.html

If a command is not in the Catalyst 4500 Series Switch Command Reference, you can locate it in the Cisco IOS library. See the Cisco IOS Command Reference and related publications at this location:

http://www.cisco.com/en/US/products/ps6350/index.html

About Storm Control

This section contains the following subsections:

• Hardware-Based Storm Control Implementation
• Software-Based Storm Control Implementation

Storm control prevents LAN interfaces from being disrupted by a broadcast storm. A broadcast storm occurs when broadcast packets flood the subnet, creating excessive traffic and degrading network performance. Errors in the protocol-stack implementation or in the network configuration can cause a broadcast storm.

Hardware-Based Storm Control Implementation

Broadcast suppression uses filtering that measures broadcast activity in a subnet over a one-second interval and compares the measurement with a predefined threshold. If the threshold is reached, further broadcast activity is suppressed for the duration of the interval. Broadcast suppression is disabled by default.

Figure 1-1 shows the broadcast traffic patterns on a LAN interface over a given interval. In this example, broadcast suppression occurs between times T1 and T2 and between T4 and T5. During those intervals, the amount of broadcast traffic exceeded the configured threshold.

Figure 1-1 Storm Control Example—Hardware-based Implementation

 

The broadcast suppression threshold numbers and the time interval combination make the broadcast suppression algorithm work with different levels of granularity. A higher threshold allows more broadcast packets to pass through.

Broadcast suppression on the Catalyst 4500 series switches is implemented in hardware. The suppression circuitry monitors packets passing from a LAN interface to the switching bus. If the packet destination address is broadcast, then the broadcast suppression circuitry tracks the current count of broadcasts within the one-second interval, and when a threshold is reached, it filters out subsequent broadcast packets.

Because hardware broadcast suppression uses a bandwidth-based method to measure broadcast activity, the most significant implementation factor is setting the percentage of total available bandwidth that can be used by broadcast traffic. Because packets do not arrive at uniform intervals, the one-second interval during which broadcast activity is measured can affect the behavior of broadcast suppression.

Enabling Broadcast Storm Control

To enable storm control, perform this task:

The following example shows how to enable storm control on interface:

Enabling Multicast Storm Control

Catalyst 4900M, Catalyst 4948E, Supervisor Engine 6-E, Supervisor Engine 6L-E, Supervisor Engine 7-E, and Supervisor Engine 7L-E support per-interface multicast suppression, which allows you to subject incoming multicast and broadcast traffic to interface-level suppression.

Note Multicast and broadcast suppression share a common threshold per interface. Multicast suppression takes effect only if broadcast suppression is enabled. Disabling broadcast suppression on an interface also disables multicast suppression.

To enable multicast suppression, perform this task:

The following example shows how to enable multicast suppression on ports that have broadcast suppression already enabled:

Disabling Broadcast Storm Control

To disable storm control, perform this task:

The following example shows how to disable storm control on interface.

Disabling Multicast Storm Control

To disable multicast suppression, perform this task:

Displaying Storm Control

Note Use the show interface capabilities command to determine the mode in which storm control is supported on an interface.

The following example shows an interface that supports broadcast suppression in software (sw):

Note Use the show interfaces counters storm-control command to display a count of discarded packets.