Why do I need an active uplink to use an appliance port?

Reader Peter sent the following question as a comment to my Direct Attach Appliance Ports post.

When I connected my NAS to use the appliance port, in order for the vnic of the blade server to communicate with the NAS, i found that there should be a connected uplink port in the 6100 even though I created a private VLAN for the appliance port and the vnic. Why?

I thought the topic would make for a nice brief post explaining the Network Control policy and its effect on Service Profile vNIC objects.

By default, all vNIC configurations use a Network Control Policy called “default” which is created automatically by UCS Manager.

The policy specifies that CDP frames are not delivered to any vNIC using this policy, and that if no uplinks are available, that the vNIC should be brought down.

In Peter’s case, since there were no uplinks available, his vNIC is kept down keeping him from using the appliance connected to the Fabric Interconnect.

If we change the policy to instead to Warning, the vNIC will be kept up (though a system warning will be generated) even when there are no available uplinks.   Note that this effectively disables the Fabric Failover feature on any vNIC using this policy.

If you have some interfaces that you want to stay up even when there are no available uplinks, create a policy with this setting and then specify it in the vNIC configuration.  Alternatively, if you want the default behavior of all vNICs (unless specifically configured) to be that they stay up even when no uplinks are available, you can modify the “default” policy as shown here.