Configure Dynamic Channel Assignment

In this post we will learn about DCA and it’s a really cool feature of RRM.

DCA is managed by RF Group Leader (How to define RF leader, we saw in one of my last post)

DCA used to determine the optimal AP channel based on these parameters.

Load: Percentage of time spent transmitting 802.11 frames

Noise: Measurement of non-802.11 signals on every serviced channel

Interference: Percentage of radio time used by neighbor 802.11 transmissions

Signal strength: Received signal strength indication (RSSI) measurement of the received neighbor messages

These values are then used by the Group Leader to determine if another channel schema will result in at least a bettering of the worst performing AP by 5dB (SNR) or in other words: Based on these metrics, if the worst performing AP will benefit by at least 5 dB or more, a channel change will take place. The decision to change the channel of an AP is also weighted to prevent a mass change within the RF group. We would not want to have a single AP change channel and have that change result in 20 other APs having to change their channel. The controller also takes into account how heavily an AP is used. A less utilized AP is more likely to have a channel change instead of a heavily used neighbor (isn’t it an interesting feature?). This helps mitigate client disassociations during a DCA event because a radio channel change disconnects all associated clients.

***Note: When an AP first boots up out the box, it transmits on channel 1 on the 802.11b/g radio and channel 36 for the 802.11a radio. The channels change according to any DCA adjustments if necessary. If a reboot occurs, the APs remain on the same channel they were using before the reboot until a DCA event occurs. If an AP is on channel 152 and reboots, it will continue to use channel 152 when it comes back up.

***Note: Radios using 40-MHz channels in the 2.4-GHz band or or 80MHz channels are not supported by DCA.

The RRM startup mode is invoked in the following conditions:

  • In a single-controller environment, the RRM startup mode is invoked after the controller is rebooted.
  • In a multiple-controller environment, the RRM startup mode is invoked after an RF Group leader is elected.

Configure DCA:

***We must disable 802.11a and b radio before changing the config. for DCA and then enable it again. Simplest way to enable/disable the radio is via CLI:

(WLAN1) >config 802.11a disable network
(WLAN1) >config 802.11a enable network

Go to Wireless > 802.11a/n or 802.11b/g/n > RRM > DCA



There is three type of Channel Assignment Method.

Channel Assignment Mode:

  • Automatic: This mode will cause the controller to periodically evaluate and, if necessary, update the channel assignment for all joined access points.
  • Freeze: It will Causes the controller to evaluate and update the channel assignment for all joined access points, but only when we click Invoke Channel Update Once.
  • OFF: Turns off DCA and sets all access point radios to the first channel of the band.

Avoid Foreign AP Interference:  It detect foreign AP and take into consideration while changing the channel.

Avoid Cisco AP Load: When its enabled then the AP load is taken into account before result in which AP will change the channel (least loaded AP will change the channel first.

Avoid Non-802.11a (802.11b) Noise: It cause the controller’s RRM algorithms to consider noise (non-802.11 traffic) in the channel when assigning channels to lightweight access points.

Avoid Persistent Non-Wi-Fi Interference:  Its enable the controller to ignore or avoid from persistent non-Wi-Fi interference.

Channel Assignment Leader: The IP address of the RF group leader, which is fully responsible for channel assignment.

Last Auto Channel Assignment: The last time RRM evaluated the current channel assignments.

DCA Channel Sensitivity: We have 3 levels (Low, Medium and High)

Channel Width:  depends on the 802.11a or b radios:  5GHz select 40MHz. In 2.4 GHz it will be 20MHz.

Avoid check for non-DFS channel: Enabled then the controller avoid checks for non-DFS channels. (Apply only for outdoor APs)

DCA Channel List: This option shows the selected channel on this radio.


Dont forget to enable both radios after changing the parameter in this section by using these commands 🙂

Radio Resource Management

This feature is very importent to understand; Resource Management (RRM) to continuously monitor the RF environment. The controller uses the information from the access points (AP) and makes any changes to AP channels and power levels to try to mitigate such things as non-802.11 signal (noise), interference from other 802.11 devices, coverage gaps, and co-channel interference caused by the network.

Here is links to understand the RRM process:

Radio Resource Management Part 1
Radio Resource Management Part 2

The RRM feature, also known as Auto-RF, uses the RF information gathered by the APs to make decisions on whether channel assignment or power levels need to be adjusted.

RRM enables controller to monitor their associated AP for the following information.

RRM Analyze

RRM Performs

  • Traffic Load
  • Interference
  • Noise
  • Coverage
  • Others AP
  • Radio resource monitoring
  • Transmit power level
  • Dynamic channel assignment
  • Coverage hole detection and correction


RRM Key Figures

RRM Neighbor messages is sent

  • At lowest mandatory speed, max power(standard according to country regulations)
  • Every 60 seconds by default
  • On all serviced channels

Important points when dealing with RRM

  • The controllers elect the RF group leader.
  • The RF group leader is responsible for dynamic channel assignment (DCA) and transmits power control (TPC).
  • An individual controller handles coverage hole detection and correction.
  • RF groups and mobility groups are independent functions.
  • RF grouping is per radio. The RF group leader for the 802.11b/g network might not be the same RF group leader for the 802.11a network.
  • With code Release 4.2.99 or higher, RRM supports up to 20 controllers and 1000 APs in a single RF group.
  • RF fluctuations can cause the RF group leader to change.
  • By default, the RF group leader polls the other controllers in the RF group for AP statistics and neighbor messages.
  • The transmit power threshold setting should be the same between all controllers in the same RF group, because you do not want an entire network to start fluctuating because of a group leader change.
  • Each AP maintain a list of up to 34 RRM neighbors per radio
  • Controller keeps the best 24 (per radio)
  • Controller forwards the list of 24 to the RF-Leader
  • Neighbor AP must be -80dBm or better to be on the list
  • An AP on the list gets dropped if its signal falls below -85dBm
  • Group leader tries to maintain -70dBm threshold between APs

Basic workflow:

  1. The controllers and their APs use the configured RF group name to determine if other APs they hear are part of their RF group.
  2. The APs use neighbor messages (sent every 60 seconds) that are authenticated by other APs that hear them. The neighbor messages include information about the AP, the controller, and the configured RF group name.
  3. The APs that hear the neighbor message of another AP authenticate that message using the RF group name and pass it to their respective controller.
  4. The controllers use this information to determine what other controllers should be in their RF group, and then form logical groups to share the RF information from their respective APs, and elect an RF group leader.
  5. The RF group leader runs the RRM algorithm against the RF information from all the APs in the RF group. Depending on the outcome, a power level or channel change for an AP or group of APs might take place.

RRM Inter-controller communication

  • Controller must be part of the same mobility group
  • RF group is a subset of mobility group
  • Max limits 20 controllers or 1000 AP per RF-Groups
  • Group Leader elected automatically based on controller id value and highest MAC address
  • Updates group member every 600 seconds for channels and power values
  • For 802.11b/g ports 12124 and 12134 are used
  • For 802.11a ports 12125 and 12135 are used.

RRM message contains:

  • Radio ID: If the AP had multiple radios, this field identifies the radio used to transmit the message.
  • Group ID:  The 16-bit value and controller MAC address.
  • Management IP address of the controller (if OATP enabled): RF group leader’s management IP address.
  • Channel Count (unused)
  • Antenna Pattern: The antenna pattern currently in use.
  • Measurement Interval
  • Key
  • Channel: The native channel that the AP uses to service clients.
  • Power (Unused)

When controllers learn of another controller from the AP neighbor messages, they communicate directly with one another to form a system-wide RF group. After the system-wide RF group is formed, the controllers elect an RF group leader.

The group leader is the controller with the highest group leader priority. The group leader priority is based on the group identifier (group ID) information element (IE) contained in the AP neighbor messages. Every controller maintains a 16-bit counter that starts at 0 and increments following events like adding or leaving an RF group or the controller being rebooted. This counter value and the MAC address of the controller make up the group ID IE. Every controller in the RF group selects one controller, or itself, that has the highest group ID value and compares this to the selected controller from the other controllers in the group. The single controller with the highest group ID is elected the RF group leader.

If the RF group leader goes offline, the entire RF group breaks up and the RF grouping process and election of an RF group leader starts over.

For an RF group to form, it takes only one AP on one controller to hear another AP on a different controller.