I wrote an article a while back regarding VLAN configuration when running vSphere ESXi on top of Cisco UCS.

A comment pointed out that all vNICs are automatically configured as trunks. I had not heard of this before, so I got into the CLI to take a look.

Here’s a VLAN configuration screen in the UCSM GUI for a sample vNIC:

Check out the running configuration for this vNIC on the underlying NX-OS CLI.

UCS-FI-A(nxos)# show run int veth782
interface Vethernet782
  description server 1/7, VNIC BARE-IPST-PROD-A
  switchport mode trunk
  untagged cos 2
  no pinning server sticky
  pinning server pinning-failure link-down
  no cdp enable
  switchport trunk allowed vlan 370
  bind interface port-channel1290 channel 782
  service-policy type queuing input org-root/org-root/ep-qos-Silver
  no shutdown

As you can see, even though we have a single VLAN checked in the GUI, the vNIC is still a VLAN trunk, and simply prunes all other VLANs off of the trunk.

This also means…

UCS-FI-A(nxos)# show int veth782 sw
Name: Vethernet782
  Switchport: Enabled
  Switchport Monitor: Not enabled 
  Operational Mode: trunk
  Access Mode VLAN: 1 (default)
  Trunking Native Mode VLAN: 1 (default)
  Trunking VLANs Enabled: 370
  Administrative private-vlan primary host-association: none
  Administrative private-vlan secondary host-association: none
  Administrative private-vlan primary mapping: none
  Administrative private-vlan secondary mapping: none
  Administrative private-vlan trunk native VLAN: none
  Administrative private-vlan trunk encapsulation: dot1q
  Administrative private-vlan trunk normal VLANs: none
  Administrative private-vlan trunk private VLANs: none
  Operational private-vlan: none
  Unknown unicast blocked: disabled
  Unknown multicast blocked: disabled

the native VLAN for this vNIC is still 1, UNLESS you select one of the radio buttons.

You know….in case you haven’t learned to double-check the underlying configuration when in doubt. Took me a while to learn that lesson.