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.
Resolution
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
Intermittent
Software features
Spanning Tree Protocol (STP)
Device connected to switch
Host
Server
Another Switch
Error message
IOS error messages
%UNICAST_FLOOD-4-DETECTED
