Cisco – Identifying an IP address conflict through a Cisco Switch

Perform a show mac address-table interface <switchport> on the switch that has the device(s) connected to it.

switch#show mac address-table int gi1/0/34
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
 132    001b.78d5.a2d7    DYNAMIC     Gi1/0/34

Then go to the router for the VLAN specified in the previous command and perform a show ip arp vlan <vlan#> | include <mac-address>.

router#show ip arp vlan 132 | include 001b.78d5.a2d7
Internet  192.168.132.234        62   001b.78d5.a2d7  ARPA   Vlan132

That will give you the IP address for the device.

Switches will know the MAC addresses, but not the IP addresses.

Switches are layer-2 devices, and the ethernet headers contain MAC addresses. A switch will create and maintain a MAC address table, which relates each MAC address with the port to which the device with that MAC address is connected.

IP is a layer-3 protocol, and IP addresses are in the packet headers, which are encapsulated inside the ethernet frame. A switch doesn't strip off the frame to look at the IP headers (router do that).

You need to relate the IP address to the MAC address. This is what ARP does. You can ARP with the IP address to discover the MAC address. Then, if your switch models have commands to do this, display the MAC address table of each switch to discover to which port the MAC address is connected. If the switches are all connected together, you may find the MAC address on a port which connects to a different switch, and you would then need to move to the switch on the connection and repeat the process until you find a host port.