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How to protect Cisco Catalyst switches from unicast flooding

Core issue

LAN switches use forwarding tables (Layer 2 (L2) tables and Content Addressable Memory (CAM) tables) to direct traffic to specific ports based on the VLAN number and the destination MAC address of the frame. When no entry corresponds to the frame destination MAC address in the incoming VLAN, the frame (unicast) is sent to all forwarding ports within the respective VLAN. This process can result in unicast flooding.


To avoid unicast flooding on Cisco Catalyst switches, create a  static CAM entry in the MAC address table for the paticular host that seems to be causing the issue. Unicast flooding can also be stopped if unicast flood blocking is configured on all the ports.

For more information about possible causes and the implications of unicast packet flooding in switched networks, refer to Unicast Flooding in Switched Campus Networks.

Problem frequency

After configuration change


Software features

Spanning Tree Protocol (STP)

Device connected to switch



Another Switch

Error message

IOS error messages


Version history
Revision #:
1 of 1
Last update:
‎06-22-2009 03:34 PM
Updated by:
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