Why Do I See set_dfs Log Messages Even When Using Non-DFS Channels?

[RobertHoTW]

I'm using a WLC with APs configured to only use non-DFS 5GHz channels (Channel 36, 40, 44, 48, 149, 153, 157, 161). In the RF Profile, I have explicitly disabled DFS channels.

WLC:C9800-L-F-K9; version:17.9.6

AP:C9120AXI-T; version:17.9.6

However, I still occasionally see log messages on the APs like the following:

Apr 7 00:00:17 WAP03 kernel: [*04/07/2025 00:00:17.3922] Enabling Channel and channel width Switch Announcement on current channel 
Apr 7 00:00:17 WAP03 kernel: [*04/07/2025 00:00:17.4002] DOT11_DRV[1]: set_dfs Channel set to 36/40, CSA count 6
Apr 7 12:00:15 WAP03 kernel: [*04/07/2025 12:00:15.1049] Enabling Channel and channel width Switch Announcement on current channel 
Apr 7 12:00:15 WAP03 kernel: [*04/07/2025 12:00:15.1129] DOT11_DRV[1]: set_dfs Channel set to 48/40, CSA count 6

Why does the system log reference "set_dfs" when the AP is using a non-DFS channel (Channel 48 in this case)?

[marce1000]

  - FYI : https://community.cisco.com/t5/wireless/9120-ap-logs/m-p/4835351/highlight/true#M255872
    You may also want to test this ,you may also want to test this against the latest advisory release ;
    https://software.cisco.com/download/specialrelease/b5170408cc94b5cb4ffce878e044cf6e

   M.

[Rich R · Accepted solution]

That download link doesn't work @marce1000 maybe it should be
https://software.cisco.com/download/specialrelease/b5170408cc94b5cb4ffce878e044cf6e
Either way it's always best to refer to the TAC recommended link below for the most up to date info.

Also you should take the contents of the logs with a pinch of salt - only the developer who wrote that software can tell you why it's mentioning dfs even when using non-DFS channels.  My guess would be that routine is checking for the need to run DFS regardless of the channel.

[RobertHoTW]

Thanks a lot for your kind reply!

[Rich R]

You're welcome.