What's that setting? Scan Defer Priority

In Cisco WLCs there is a setting called Scan Defer Priority. I've often seen this when performing configurations and I've been curious about modifying it, but never curious enough to do the research. I figure there must be others out there with the same curiosity, so let's dig in and figure this out together.

Below is a screenshot of the Scan Defer Priority settings under the Advanced Tab of the SSID Configuration Page in my Lab Controller.

A quick search turns up the following link on Cisco's website, so let's start there and see where we land.

Here we find that the setting directly relates to off channel scanning as it is used for Radio Resource Management (RRM). If you are unfamiliar, RRM is the name for the software function that performs Dynamic Channel Assignment (DCA) and TPC (Transmit Power Control). This Off Channel Scanning is a requirement for the controller to detect conditions in the RF environment that necessitate either moving the AP to a new channel (to avoid interference or rogues), or adjusting the power to the radio based on the result of the scanning.

In the example above, the checked boxes are in reference to user priority, and the timer is a relative value based on the last time traffic was seen for the particular user priority associated with the traffic.

Worth mentioning on the Cisco page is a line stating the default setting (enabled for 4,5,6) should protect voice traffic. When the radio function is performing off-channel scanning, it's not servicing client traffic. This is a trade-off that administrator's should be aware of when enabling RRM. Some applications may not work well with the radio being unavailable to serve client traffic for the 80ms of scanning every 3-20 seconds.

Cisco specifies a callout that even if 2.4GHz scanning is not selected, but there is Bluetooth BLE support, then the 2.4GHz radio will still perform scanning to support the Bluetooth features. There is an explicit configuration option to disable this as well.

Having read through this, I felt like I started getting the grasp of this, but I still need more information. The primary question I have here is:

What is user priority and how is it determined?

According to this whitepaper at Cisco User Priority (UP) is the 80211e QoS value attached to the wireless traffic. The default settings here are used to protect latency sensitive voice and video traffic on the WLAN.

So what should an engineer do with this setting?

In my opinion, sticking with the default settings if you have RRM enabled should be your initial stance. If you aren't doing anything with Bluetooth (Think IoT) you could likely disable the BLE scanning option, but be sure to document this as your network requirements may change over time to demand this feature. If you are hard-coding your channels and transmit power, this feature should not be performing any action.

As an aside, I am a little disappointed that Cisco does not refer to the 80211e priority by name in this window, or make a direct mention of the UP setting by name in the GUI as it is called out in the Configuration Guide. As a leading manufacturer, I would expect to see configuration items reference their IEEE working group amendment.