Cisco Switch Port-Security – MAC Address Table Showing Same MAC Address on One Port

ciscomacport-securityswitch

When I try the sh mac address-table int gi 1/0/19 command:

sh mac address-table int gi 1/0/19
          Mac Address Table
 -------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
  20    38ed.1855.ed28    DYNAMIC     Gi1/0/19
  30    38ed.1855.ed28    DYNAMIC     Gi1/0/19
Total Mac Addresses for this criterion: 2

I connected an IP phone on VLAN 30 and a PC on VLAN 20. The switch is a Cisco 2960 48-port, IOS version 15.0(2a)EX5.

Best Answer

Your Cisco VoIP phone is also a switch which negotiates a trunk link with the switch to which it connects. It shows up on both VLANs because it is on both VLANs. This is normal behavior.

This is the reason you need to be careful when configuring port security on your switch. You would probably need to allow three connections, two for the phone, and one for the PC connected to the phone.

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