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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter22 Configuring Port-Based Traffic Control
Configuring Storm Control
Understanding Storm Control
A packet storm occurs when a large number of broadcast, unicast, or multicast packets are received on
a port. Forwarding these packets can cause the network to slow down or to time out. Storm control is
configured for the switch as a whole but operates on a per-port basis. By default, storm control is
disabled.
Storm control uses rising and falling thresholds to block and then restore the forwarding of broadcast,
unicast, or multicast packets. You can also set the switch to shut do wn t he p ort when the rising threshold
is reached.
Storm control uses one of these methods to measure traffic activity:
•Bandwidth based
•Traffic rate at which packets are recei ved ( in packets p er second) (a vai lable only o n non-Long-Rea ch
Ethernet [LRE] Catalyst 2950 switches)
The thresholds can either be expressed as a percentage of the total available bandwidth that can be used
by the broadcast, multicast, or unicast traffic, or as the rate at which the interface receives multicast,
broadcast, or unicast traffic.
When a switch uses the bandwidth-based method, the rising threshold is the percentage of total a v ailable
bandwidth associated with multicast, broadcast, or unicast traffic before forwarding is blocked. The
falling threshold is the percentage of total available bandwidth below which the switch resumes normal
forwarding. In general, the higher the level, the less effective the protection against broadcast storms.
When a non-LRE Catalyst 2950 switch running Cisco IOS Release 12.1(14)EA1 or later use s traffic rates
as the threshold values, the rising and falling thresholds are in packets pe r second. The rising threshold
is the rate at which multicast, broadcast, and unicast traffic is received before forwa rding is blocked . The
falling threshold is the rate below which the switch resumes normal forwarding. In general, the higher
the rate, the less effective the protection against broadcast storms.
Default Storm Control Configuration
By default, broadcast, multicast, and unicast storm control is disabled on the switch. The default action
is to filter traffic and to not send an SNMP trap.
Enabling Storm Control
Beginning in privileged EXEC mode, follow these steps to enable storm control:
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 interface interface-id Specify the port to configure, and enter interface configuration
mode.