Network troubleshooting, Part II: What physical switch port does the pNIC connect to?

When you have found out what physical NIC (pNIC) a VM is actually using (see my previous post) you may want to check the external switch port that this pNIC connects to (Is it properly configured, what about the error counters?). Okay, what switch port do you need to check?
It is considered good data center practice to have every connection from every server's pNIC to the switch ports carefully documented. Do you? Do you trust the documentation? Is it up to date? If yes you are fine and can stop reading here...

If you want to be sure, and if you use Cisco switches in your data centers then there is a much more reliable way to track these connections: The Cisco Discovery Protocol (CDP). On Cisco devices this is enabled by default, and it periodically broadcasts interface information to the devices attached to its ports (like your ESX hosts).
By default ESX(i) (version 3.5 and above) will receive these broadcasts and display the information in it through the VI client. In the Hosts and Clusters-view select Networking in the Configuration tab of a host. This will display your virtual switches with their physical up-links (vmnic0, vmnic1, etc.). Now click on the little speech bubbles next to the Physical Adapters and a window like the following will pop up:

CDP information shown in the VI client

You can find a lot useful information here. The Device ID is the name of the Cisco switch. And Port ID shows the number/name of the switch module and the port number on that module. So you can tell your network admins exactly what switch port they need to check.

If CDP information is not available for a physical adapter the pop-up window will also tell you this. Possible reasons: You don't use Cisco switches or have CDP broadcasts disabled on them, or the ESX(i) host's interfaces are not in CDP listen mode.

For more detailed information on CDP and how to configure it in ESX see the VMware KB: Cisco Discovery Protocol (CDP) network information.