some access points C9120AXI-S are using 20MHz channel width

[Andrey128]

On 9800L WLCs there are several access points C9120AXI-S, which are using 20MHz channel width instead of configured global value 40 MHz.

Is it a bug or a feature ?!

[marce1000]

  - Yet your screenshot indicated that the Channel Width is set to 20Mhz ?

 M.

[Andrey128]

screen is just shows that mentioned access point with 20 MHz channel width is using "global" channel width assignment method (the same as other access points) and for some reason it is 20MHz (grayed / inactive in WEB interface as it is from global settings) instead of 40MHz  (grayed / inactive in WEB interface as per expectation) , as most other access points have

sorry if confused screens

[marce1000]

 - Ok ,  check the 9800   configuration with the CLI command : show  tech   wireless , have the output analyzed by  https://cway.cisco.com/tools/WirelessAnalyzer/  , please note do not use classical show tech-support (short version) , use the command denoted in green for Wireless Analyzer.               Checkout all advisories !

 M.

[Rich R · Accepted solution]

It's neither a bug nor a feature really - it's just the way channel assignment works.  The channels are paired and sometimes you have a channel in the DCA list which has no bonded channel partner to be used so if that channel is selected then it just uses the single channel at 20MHz.  If you don't want that to happen take those single channels out of your DCA channel list.  There are also some cases where it's not possible to use the 2nd channel so you get the same effect.  I shared a link explaining this on another post some months ago - found it: https://www.cwnp.com/the-wise-article-series-wi-fi-subtleties-explained-channel-bonding/

[Arshad Safrulla · Accepted solution]

Since the problematic AP is channel 124 it is possible that the AP saw a DFS event in channel 128 and then RRM process removed channel 128 from assignment.  I would also make sure that the DCA is configured properly in the WLC.

ap dot11 5ghz rrm channel dca chan-width 40

ap dot11 5ghz rrm channel dca chan-width width-max WIDTH_40Mhz

If you are configuring the channel width under the RF policy then you can follow below 

ap dot11 5ghz rf-profile Test-RF-P
channel chan-width 40