Cisco Catalyst 9800 Series Wireless Controller Software Configuration Guide, Cisco IOS XE 26.1.x

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Cisco Catalyst 9800 Series Wireless Controller Software Configuration Guide, Cisco IOS XE 26.1.x

Dynamic frequency selection

Updated: April 10, 2026

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Introduces Dynamic Frequency Selection concepts, details Channel Availability Check history and operation, and provides instructions for configuring DFS using GUI and CLI methods with verification procedures.

Dynamic Frequency Selection (DFS) is a process that

  • detects radar signals on DFS-enabled 5.0-GHz (802.11a/h) radios

  • automatically sets the frequency to avoid interference with radar signals, and

  • ensures radios configured for use in a regulatory domain do not interfere with radar systems.

Flex DFS behavior

In normal DFS, when a radar signal is detected on any of the channels in the 40-MHz or 80-MHz bandwidth, the whole channel is blocked. With Flex DFS, if the radar signals are not detected on the secondary channel, the AP is moved to a secondary channel with a reduction in the bandwidth, usually, by half.

Feature history for channel availability check (CAC)

This table provides release and related information for features explained in this module. These features are available on all releases subsequent to the one they were introduced in, unless noted otherwise.

Table 1. Feature history for channel availability check (CAC)

Release

Feature

Feature Information

Cisco IOS XE Bengaluru 17.5.1

Channel Availability Check (CAC)

When a DFS channel is selected for an AP radio, the AP radio scans the channel to check for any radar signals before transmitting any frames in the DFS frequency. This process is called Channel Availability Check (CAC).

Channel availability check

A channel availability check is a DFS channel scanning process that

  • scans DFS channels for radar signals before transmitting any frames in the DFS frequency

  • executes before you set a DFS channel for the radio, and

  • marks channels as non-available for 30 minutes if radar is detected.

Channel availability check behavior

When a DFS channel is selected for an AP radio, the AP radio scans the channel to check for any radar signals before transmitting any frames in the DFS frequency. This process is called Channel Availability Check (CAC).

Note

CAC is executed before you set a DFS channel for the radio.

If the AP detects that a radar is using a specific DFS channel, the AP marks the channel as non-available and excludes it from the list of available channels. This state lasts for 30 minutes after which the AP checks again to see, if the channel can be used for Wi-Fi transmissions.

Note

  • The CAC performed during a boot process takes anywhere between 1 and 10 minutes depending on the country. This is the reason as to why the DFS channels are not available immediately when an AP reboots.

  • APs in the ETSI domain scan channels which are not supported by the controller, as the hardware has the ability to scan.

DFS verification

Use the following commands to verify the DFS configuration:

To display the 802.11h configuration, use the following command:

Device# show wireless dot11h

To display the auto-rF information for 802.11h configuration, use the following command:

Device# show ap auto-rf dot11 5ghz

To display the auto-rF information for a Cisco AP, use the following command:

Device# show ap name ap1 auto-rf dot11 5gh

To display the channel details for a Cisco AP, use the following command:

Device# show ap dot11 5ghz summary
AP Name Mac Address Slot Admin State Oper State Width Txpwr Channel 
-----------------------------------------------------------------------------------------------------------------------
pnp-ap 04eb.409e.b560 1 Enabled Up 40 *8/8 (3 dBm) (52,56) 
BLDG1-9130-RACK-1568 04eb.409f.11a0 1 Disabled Down 40 4/8 (15 dBm) (100,104)#
Note

In the show command, # is added right next to the channel whenever CAC is running on an AP radio.