Beware VLAN double tagging!

In setting up some additional ESXi hosts in an aforementioned lab we ran into an issue where we could not communicate with the new hosts after setting static IPs and relevant management VLANs on them. The hosts are connected to 2 TOR switches (Cisco 9K Top Of Rack). Investigating on the switch you could see the hosts connected on the expected port on each switch (Ethernet 1/14 on each) by searching the mac address table for the relevant mac

and that the required VLAN 2556 was trunked to those ports.

Digging a little deeper we discovered that the switch ports had a native VLAN of 2556 set so we were falling victim to double tagging of this VLAN, as we were also tagging at the ESXi host level.

We want our hosts tagged so the simple fix was to remove the native VLAN tag on the switch ports

First enter config mode

  • conf t

Then enter interface config mode

  • int Ethernet 1/14

Then disable the native vlan

  • no switchport trunk native vlan 2556

Our ESXi hosts are now happily communicating in and out! The morale of the story?? Beware double tagging of VLANs!

 

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