storm-control broadcast level [pct]
Configures broadcast storm control on a switch interface to limit the percentage of broadcast traffic, preventing network disruptions from excessive broadcasts.
storm-control broadcast level [pct]When to Use This Command
- Limiting broadcast traffic on an access port connected to a user segment to prevent broadcast storms from affecting the entire VLAN.
- Protecting a trunk link from broadcast storms originating from a misconfigured or malicious device in another VLAN.
- Controlling broadcast traffic on a port connected to a server that generates high broadcast rates, ensuring stability for other devices.
- Enforcing traffic policies on a port connected to a legacy device that may send excessive broadcasts due to bugs or misconfiguration.
Command Examples
Setting broadcast storm control to 50% on an interface
interface GigabitEthernet0/1
storm-control broadcast level 50Switch(config-if)#storm-control broadcast level 50 Switch(config-if)#
The command sets the broadcast storm control threshold to 50% of the interface bandwidth. No output is displayed upon successful configuration; the prompt returns without error.
Verifying storm control configuration
show storm-control broadcastInterface Filter State Upper Lower Current --------- ------------ ---------- ---------- ---------- Gi0/1 Forwarding 50.00% 50.00% 0.00% Gi0/2 Blocking 30.00% 20.00% 35.00%
Interface: The interface name. Filter State: 'Forwarding' means traffic is passing; 'Blocking' means storm control is actively dropping traffic. Upper: The rising threshold (percentage of bandwidth) that triggers storm control. Lower: The falling threshold (percentage) at which storm control stops dropping traffic. Current: The current broadcast traffic percentage. In this output, Gi0/1 is forwarding with 0% broadcast traffic, while Gi0/2 is blocking because current (35%) exceeds the upper threshold (30%).
Understanding the Output
The 'show storm-control broadcast' command displays per-interface storm control status. The 'Filter State' indicates whether the interface is forwarding or blocking broadcast traffic. 'Upper' and 'Lower' are the configured thresholds; typically they are set to the same value unless a hysteresis range is configured. 'Current' shows the real-time broadcast traffic percentage. A healthy interface shows 'Forwarding' with a current value below the upper threshold. If 'Current' exceeds 'Upper', the interface enters 'Blocking' state and drops broadcast traffic until 'Current' falls below 'Lower'. Watch for interfaces that frequently toggle between states, indicating a potential broadcast storm issue. Also, note that storm control only affects broadcast, multicast, and unknown unicast traffic; normal unicast forwarding is unaffected.
CCNA Exam Tips
CCNA exam tip: Storm control thresholds are percentages of interface bandwidth, not absolute values. Remember that the default action is to filter (drop) excess traffic, not to send SNMP traps unless configured.
CCNA exam tip: You may be asked to configure storm control on a specific interface to limit broadcast traffic to 20% of bandwidth. Use 'storm-control broadcast level 20' in interface configuration mode.
CCNA exam tip: The 'show storm-control' command can verify the configuration. Know that 'Filter State' can be 'Forwarding' or 'Blocking'.
CCNA exam tip: Storm control is a Layer 2 feature; it does not apply to routed ports unless configured as switchports.
Common Mistakes
Mistake 1: Forgetting to enter interface configuration mode before applying the command, resulting in a global configuration error.
Mistake 2: Setting the threshold too low (e.g., 1%) causing legitimate broadcast traffic (like ARP) to be dropped, disrupting network operations.
Mistake 3: Not verifying the configuration with 'show storm-control', leading to undetected misconfiguration.
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