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:

ucs_vlan

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.


Matt Oswalt

Matt Oswalt is an all-around technology nerd, currently focusing on networking, software development, and everything in between. He is at his happiest in front of a keyboard, next to a brewing kettle, or wielding his silo-smashing sledgehammer. He spends his days diving deep into the intersection of networking and software, and likes to blog about his experiences when he comes up for air. You can follow him on Twitter or LinkedIN.