11-1. Broadcast Suppression A network protocol can create a large amount of broadcast traffic. In Layer 2 networks, broadcasts must be forwarded on all ports except the receiving port; because of this, a large or excessive number of broadcasts can have an impact on network and device performance. Broadcast suppression enables you to control how a receiving port handles excessive broadcast traffic. By configuring a threshold, a port can be configured to stop flooding broadcasts for a predefined period or until the broadcasts fall below a certain level. Suppressing these broadcasts will prevent them from being forwarded out other switch ports and limit the effect they have on the network. Suppression of the broadcasts does not have any effect on the multicast or unicast traffic received by the port. Broadcast suppression is supported by both the COS and IOS operating systems on most Catalyst platforms. In addition to broadcast suppression, unicast and multicast suppression can also be configured for some platforms.
Configuring Broadcast Suppression By default broadcast suppression is disabled on all platforms and on all operating systems. Broadcast suppression is applied to individual ports on a switch. When configuring broadcast suppression, keep in mind that it is the number of broadcasts received by a port. When the threshold is reached, the port stops passing broadcast packets to the backplane until the condition is corrected. To configure broadcast suppression, use the following steps. 1. | Enable broadcast suppression:
COS | set port broadcast mod/port threshold%
| 3500XL IOS | [View full width] (interface) port storm-control broadcast threshold  rising ppsrisingthreshold# falling ppsfallingthreshold#
| 2950 IOS | [View full width] (interface) storm-control broadcast level risingthreshold% [fallingthreshold%]
| 3550 IOS | (interface) storm-control broadcast level threshold%
| Supervisor IOS (Catalyst 6000) | (interface) broadcast suppression threshold%
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The syntax for broadcast suppression, and the action, varies greatly by platform. For the COS switches, setting a threshold% on the interface to less than 100 percent enables broadcast suppression. The threshold limits the amount of bandwidth on an interface that can be used for broadcast packets. For example, a value of 50 percent means that the interface is required to take action on all broadcast packets that were above 50 percent of the total bandwidth. A value of 100 percent disables broadcast suppression because it lets the broadcast take up to 100 percent of the available bandwidth. For the 2950, 3550, and Supervisor IOS on the 6000 series switch, the concept is the same. The threshold% or risingthreshold% specifies the percentage of bandwidth limit that would have to be reached by broadcast traffic before action would be taken.
For the 2900/3500XL switches, the rising value that causes broadcasts to be suppressed is a packet per second (pps) value. When the ppsrisingthreshold# is reached, action is taken against the packets. For these switches a ppsfallingthreshold# must also be configured. This is the value the broadcast must fall below before the action is removed from the port. You might want to keep in mind that a 100 mbps Ethernet port has a maximum forwarding capability of 148,000 pps in full-duplex mode. Any value of 148,000 pps or more would have no effect on controlling broadcasts.
On the COS switches, on all operating systems on the 6000 series, and on the 3550 switch, the suppression is a time-based actionthat is, broadcasts are suppressed over a 1-second interval. On the 2950 and 2900XL switches, the suppression is based on an absolute value or a falling value. The fallingthreshold is an optional value that can be set to specify when the action is removed. If the risingthreshold is 50 percent and the fallingthreshold is 45 percent, for example, when 50 percent of the traffic is broadcast, action is taken until the value falls below 45 percent. Because the falling value is not required, and if no falling value is set, the default is to remove the action when the percentage of broadcast traffic falls below the rising value.
NOTE Broadcast suppression is not supported on the Catalyst 4000 series switch. | 2. | Specify action to be taken:
COS | [View full width] set port broadcast mod/port threshold% [violation {drop-packets | errdisable}]
| 3500XL IOS | [View full width] (interface) port storm-control broadcast action {filter | shutdown}
| 2950 IOS | [View full width] (interface) storm-control broadcast action {shutdown | trap}
| 3550 IOS | N/A | Supervisor IOS (Catalyst 6000) | N/A |
When broadcast suppression occurs, the default action is to suppress or filter the packets. This means that packets are dropped and do not make it onto the backplane of the switch. You can, on some of the platforms, configure the switch to take another action. For example, on COS switches you can place the port in an Errdisable state. This means that the port will stay in this state as long as the threshold is reached and even after that until an administrator has corrected the problem. For the 3500XL switch, you can change the device from the default action of filter to the shutdown option. When the port is placed in shutdown mode, it remains there until an administrator has reenabled the port with the no shutdown command. Each time the threshold is crossed, the administrator must reenable the port. For the 2950 broadcast, frames are dropped unless the action has changed to shutdown; this works the same as it does on the 3500XL. To revert to filtering the frames, the administrator must issue the command no port storm-control broadcast action shutdown. Another option that can be configured on the 2950 is for the switch to generate an SNMP trap. The action is not configurable for the 3550 or the Catalyst 6000 running Supervisor IOS.
NOTE You can have the switch automatically try to enable a port that has been placed in an Errdisable state because of a broadcast storm using the command set errdisable-timeout enable bcast-suppression. | 3. | (Optional) Control unicast or multicast:
COS | [View full width] set port broadcast mod/port threshold% [multicast {enable | disable}] [unicast {enable | disable}]
| 3500XL IOS | [View full width] (interface) port storm-control {multicast | unicast} threshold rising ppsrisingthreshold# falling ppsfallingthreshold#
| 2950 IOS | [View full width] (interface) storm-control {multicast | unicast} level risingthreshold% [fallingthreshold%]
| 3550 IOS | [View full width] (interface) storm-control {multicast | unicast} level threshold%
| Supervisor IOS (Catalyst 6000) | N/A |
In addition to configuring the switch to control broadcast floods, you can also configure a port to drop frames or become disabled when it encounters a large number of unicast or multicast packets. To configure this option, use the multicast and unicast keywords in the commands to enable the control of the frames.
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Verifying Configuration After you have configured broadcast suppression, use the following commands to verify the configuration and operation on the switch: COS | show port broadcast [mod][/port]
| 3500XL IOS | (privileged) show port storm-control [interface]
| 2950/3550 IOS | [View full width] (privileged) show storm-control [interface] [{broadcast | multicast | unicast | history}]
| Supervisor IOS (Catalyst 6000) | [View full width] (privileged) show interfaces switchport [module number]
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Feature Example This example shows a typical configuration for setting broadcast suppression on port 3/1 of a COS switch using a threshold of 33 percent. When the threshold is met, the port is errdisabled and the switch is configured to attempt to reenable the port automatically. An example of the Catalyst OS configuration follows: Catalyst (enable)> set port broadcast 3/1 33% violation errdisable Catalyst (enable)> set errdisable-timeout enable bcast-suppression
For a 2950, this example shows a configuration that enables broadcast suppression when the traffic reaches 55 percent on interface Fast Ethernet 0/9 and resumes normal broadcast propagation when the broadcast falls below 44 percent. An example of the Supervisor IOS configuration follows: 2950(config)#interface fastethernet 0/9 2950(config-if)#storm-control broadcast level 55 44 2950(config-if)#end 2950#copy running-config startup-config
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