Configuring Broadcast Suppression - Cisco Catalyst 2000 Configuration Handbook

168 Cisco LAN Switching Configuration Handbook
11-1: Broadcast Suppression
A network protocol can create a large amount of broadcast traffic.
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■ 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
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broadcast traffic.
By configuring a threshold, a port can be configured to stop flooding broadcasts for
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a predefined period or until the broadcasts fall below a certain level.
■ Suppressing these broadcasts can 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
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traffic received by the port.
■ Broadcast suppression is supported on Catalyst IOS platforms.
In addition to broadcast suppression, unicast and multicast suppression can also be
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configured for some platforms.

Configuring Broadcast Suppression

By default broadcast suppression is disabled on all platforms and on all operating sys-
tems. 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.
Enable broadcast suppression:
1.
(interface) broadcast suppression threshold%
Broadcast suppression prevents LAN interfaces from being disrupted by a broadcast
storm. A broadcast storm occurs when multiple copies of broadcast or multicast
frames flood the subnet, creating excessive traffic and degrading network perform-
ance. Errors in the protocol-stack implementation or in the network configuration
can cause a broadcast storm.
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 sup-
pression on the Cisco 6500 series switches is implemented in hardware. The suppres-
sion circuitry monitors packets passing from a LAN interface to the switching bus.
Using the Individual/Group bit in the packet destination address, the broadcast sup-
pression circuitry determines if the packet is unicast or broadcast, keeps track of the