How to find the port a device is connected to, based on IP, on a Cisco Catalyst switch

Published on by dellpe

Question:

On a Cisco WS-C3750X-24S-S Catalyst switch, I need to find what port an end device is connected to.

I have the IP address/host name.

How do I quickly find the port?

Answer:

The answer depends on whether the switch is a Layer 2 or a Layer 3 switch. That is to say, is the switch only switching and relaying traffic on to a different device for routing, or, is it doing the routing decisions itself via SVIs (switched virtual interfaces).

On a layer 3 switch, the port can be found by using a few simple commands on the device. However on a layer 2 switch, you have to log into both the switch and whatever device is doing the routing to locate the port.

In either case, the commands are the same, just run on two different boxes for the layer 2 switch.

On a Layer 3 switch:

  • Log into the switch and issue the following command (where ipaddress is the ip address of the host you are trying to locate:

show ip arp ipaddress

  • The output should look similar to below, and give you the mac-address of the device (I've highlighted the mac-address below in bold).

LYKINS-1861#show ip arp 172.20.1.100
Protocol Address Age (min) Hardware Addr Type Interface
Internet 172.20.1.100 0 28cf.da1d.1b05 ARPA Vlan10

  • Now issue one of the following commands (where mac-address is the hardware address from previous step). Depending on the Cisco platform, sometimes the command is listed in either form.

show mac address-table address mac-address
show mac-address-table address mac-address

  • The output should look similar to below, the port you are looking for should be listed as the Destination Port:

LYKINS-1861#show mac-address-table address 28cf.da1d.1b05
Destination Address Address Type VLAN Destination Port
------------------- ------------ ---- --------------------
28cf.da1d.1b05 Dynamic 10 FastEthernet0/1/1

On a Layer 2 switch:

  • Find out what device is doing the routing for this switch (you may have to look at the network documentation). Sometimes it is a "Router on a Stick", where the Layer 2 VLANs are being trunked up to the router for the Layer 3 decisions.

  • Log into the routing device, and issue the following command (where ipaddress is the ip address of the host you are trying to locate:

show ip arp ipaddress

  • The output should look similar to below, and give you the mac-address of the device (listed below in bold).

LYKINS-1861#show ip arp 172.20.1.100
Protocol Address Age (min) Hardware Addr Type Interface
Internet 172.20.1.100 0 28cf.da1d.1b05 ARPA Vlan10

  • Now log into the Layer 2 switch
  • On that device issue one of the following commands (where mac-address is the hardware address from previous step). Depending on the Cisco platform, sometimes the command is listed in either form.

show mac address-table address mac-address
show mac-address-table address mac-address

  • The output should look similar to below, the port you are looking for should be listed as the Destination Port:

LYKINS-1861#show mac-address-table address 28cf.da1d.1b05
Destination Address Address Type VLAN Destination Port
------------------- ------------ ---- --------------------
28cf.da1d.1b05 Dynamic 10 FastEthernet0/1/1

Published on Cisco Switch

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