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On a Cisco 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?

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3 Answers

up vote 14 down vote accepted

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

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up vote 6 down vote

Firstly, you need to get the MAC address, so get into a machine on the same VLAN and look at its neighbour table - Windows is netsh int ipv4 show neigh, Linux: ip nei Cisco: show ip arp x.x.x.x. Once you have that...

If this is a discovery job on a layer 2 switch, do show mac address-table | i 0011.2233.4455 - replacing the mac address bytes as appropriate.

If on the other hand it's a router, use show ip arp | i 0011.2233.4455 - again replacing the MAC as appropriate. Obviously when you were getting the MAC, if it turned out to be directly connected to that router, you're already done.

Long-term however, I heartily recommend that you setup LLDP (failing that, CDP) to your hosts so that you can identify them from either side. lldpd is an absolutely excellent LLDP daemon for Linux that also supports CDP, EDP, SONMP and FDP. If you're currently able to reach the host and it does happen to run linux/BSD, I'd recommend skipping the above and just turn on LLDP.

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+1 for LLDP, it's a nice solution that helps from both sides. – LapTop006 May 19 at 3:09
up vote 4 down vote

Regardless of whether your switch is performing frame forwarding (layer 2) or packet routing (layer 3), the following should work if the switch has a management IP address in the same subnet as the host you want to find:

  1. At the switch, ping the IP address you want to find. If the address is on the same subnet as the switch's management address, an ARP request will be sent looking for the MAC address of the host.
  2. Now you can look at the ARP cache and find the MAC address of the host you are looking for (show ip arp) and its corresponding interface.
  3. You can also look for its MAC address table entry by issuing: show mac-address table address [the address]
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