Last week at a customer I was asked to have a look at a issue that they recently been having regarding the Distributed Switch health check feature.
This customer uses this feature to regularly check if all physical network connections are providing all the VLAN's configured on the Distributed Switches in their vSphere environment. About two weeks ago the health check suddenly started notifying errors on all hosts connected to a specific Distributed Switch. The error was a missing VLAN and a MTU size mismatch, the VLAN missing on the physical switch ports which connected to the physical NIC's of the hosts.
When I looked at the details of the error, I saw the missing VLAN was VLAN 0. This got me interested as usually you don't use VLAN 0 for standard network traffic and therefore you don't see it often in vSphere on a Distributed Switch which handles Virtual Machine network traffic.
When I checked the Distributed Switch configuration, more specific the dvPortgroups configured on it I found a dvPortGroup named "name_test" there where no VM's using it and when I looked at it's configuration I found it had the VLAN type set to "None".
After asking I learned that this dvPortGroup was created as a test during troubleshooting an other issue, after troubleshooting it was left in place. So there was no reason for leaving it there, I removed this dvPortGroup and after a refresh I checked the health status of this Distributed Switch.
To no surprise the health check showed up without any error's this time, so not only the missing VLAN error got solved but also the MTU size mismatch got solved in this.
So my take away on this is, when you as a vSphere Admin use the Distributed Switch health check feature to keep tabs on the status of your virtual to physical network please keep a tight procedure when it comes to changes on the Distributed Switches! And if you have to add a dvPortGroup for testing purposes please do not add it with the VLAN type set to "None", but add it with a VLAN that is available on your physical network interfaces.