Field Notice: FN74035 - Cisco Access Points May Not Detect Radar on the Required Levels After Channel Availability Check Time - Software Upgrade Recommended

A software defect was found in Cisco AireOS Software releases 8.3.130.0 and later and in Cisco IOS XE Software releases 16.12.1 and later.

Dynamic Frequency Selection (DFS) radar detection may not function as intended in the following Cisco products when they are operating normally and are running an affected release of Cisco AireOS Software or Cisco IOS XE Software:

• 6300 Embedded Services Access Points (APs)
• Aironet 1560 Series Outdoor APs
• Aironet 2800 Series APs
• Aironet 3800 Series APs
• Aironet 4800 Series APs
• Catalyst IW6300 Heavy Duty Series APs

APs that are running affected software may not function as intended. When an AP that is running affected software uses a DFS channel for its 5-GHz radio, the in-service monitoring (ISM) process after the initial Channel Availability Check (CAC) time (one minute or 10 minutes, depending on the channel number) may fail to detect radar properly, leading to false negatives.

Note: During the initial CAC, APs that are running affected software are able to detect radar that is present on the channel properly and react accordingly.

IMPORTANT: For more information on the issue described in this Field Notice, see this two-minute video.

All APs must adhere to the technical conditions imposed by regulators with respect to radio emissions. One technical requirement around DFS is that APs that operate in portions of the 5-GHz band listen for civil and military radars. When radar patterns are detected, APs must stop transmissions and change channels to avoid co-channel operation with these radars.

Wireless APs must perform an initial CAC for 60 seconds before using a DFS channel.

If the CAC is completed successfully and no radar is detected, then the AP will start using the selected DFS frequency while initiating new, continuous monitoring of the radio waves to detect any radar signals that may appear on the operating frequency.

In the affected software, the 60-second CAC operates as intended. However, during the continuous monitoring that begins after the 60-second CAC, the AP's ongoing detection capability may not adhere to technical regulations.

Software made available after September 2017 has impacted the ability of specific devices to detect radar events as intended. All customers, regardless of location, should take immediate action to implement the solution or workaround described in this Field Notice.

Customers will only be affected by this issue if they meet both of the following criteria:

• The customer operates APs that are running affected software.
• The customer's APs operate on DFS channels now or in the future.

There are no customer-visible symptoms.

Upgrade to a fixed software release and/or apply the AP Service Pack (APSP) patch based on the guidelines in the following table:

Note: The software recommendations listed in the table also apply to Cisco Mobility Express and Cisco Embedded Wireless Controllers on Catalyst APs.

Note: Follow the proper upgrade path as detailed in related Release Notes documents.

IOS XE Software

AireOS Software

Mobility Express Software

Cisco recommends that customers upgrade to the appropriate fixed software release and/or APSP and only implement an interim workaround until the fixed software can be installed.

Interim Workaround for Cisco IOS XE Software

To change the global channel list, use the following steps:

1. In the management GUI, choose Configuration > RRM > 5 GHz Band > DCA.
2. Under Auto-RF Channel List, uncheck check boxes 52 to 140 to disable the corresponding channels.
   Note: This change may impact all other APs on the controller.

To disable DFS channels for a specific RF profile so that an AP operates on non-DFS channels only, use the following steps:

1. In the management GUI, choose Configuration. Under Tags & Profiles, choose RF/Radio.
2. Click a 5-GHz RF profile.
   Note: All RF profiles without "pre-configured" in the description are editable. If you are using a pre-configured RF profile, replace with a custom RF profile.
3. Choose RRM > DCA. In the DCA channels list, uncheck check boxes 52 to 140 to disable the channels.

For more information on using RF profiles and tags and customizing channel lists for individual APs, see https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/17-9/config-guide/b_wl_17_9_cg/m_config_model.html#task_k2c_nps_xfb.

Interim Workaround for Cisco AireOS Software

To change the global channel list, use the following steps:

1. In the management GUI, choose Wireless > 802.11a > RRM > DCA.
2. In the DCA channel list, disable channels 52 to 140.
   Note: This change may impact all other APs on the controller.

To use RF profiles to limit channel changes to only the affected APs, use the following steps:

1. In the management GUI, click Wireless. In the menu on the left, choose RF Profiles.
2. Click a profile.
   Note: All profiles with a radio policy of 802.11a are editable.
3. Choose RRM.
4. In the DCA channel list, disable channels 52 to 140.

For more information on using RF profiles and tags and customizing channel lists for individual APs, see https://www.cisco.com/c/en/us/td/docs/wireless/controller/8-10/config-guide/b_cg810/radio_resource_management.html#rf-profiles.

Note: This workaround may not be possible in some deployment scenarios, such as on outdoor APs in some regulatory domains. This workaround will reduce the number of available channels and is not advisable in a high-density scenario.

Note: For Cisco products, the DFS channels are in 5250 to 5350 MHz (channels 52-64) and in 5470 to 5725 MHz (channels 100-144).

To find an AP product identifier (PID), use one of the following methods:

CLI

• In the Cisco AireOS Software CLI or the Cisco IOS XE Software CLI, enter the show ap summary command.
• In the AP CLI, enter the show inventory command.

GUI

• From the Cisco AireOS Software Advanced GUI, choose Wireless > All APs.
• From the Cisco Mobility Express GUI, choose Monitoring > Access Points. See the Add Model column.
• From the Cisco IOS XE Software  GUI, choose Monitoring > Wireless > AP Statistics.

Visual Inspection

For further questions related to this issue, please send an e-mail to: fn74035_questions@cisco.com