CTS 104: Cisco DCA

What is Cisco DCA and how does it work with RRM.

This episode is sponsored by Metageek

Sponsored by Metageek

Cisco RRM DCA – Dynamic Channel Assignment

DCA, or Dynamic Channel Assignment, is a core component of Cisco Radio Resource Management (RRM). It runs a critical algorithm which dynamically changes an access points (AP) client serving channel based on multiple parameters, collectively called a Cost Metric. To really get an understanding of Cisco RRM, start with our previous episode on Cisco NDP.

DCA is configured under 802.11a/n/ac (5 GHz) and 802.11b/g/n (2.4 GHz)

Configuring DCA for 802.11a/n/ac

Channel Assignment Method

The first thing you will notice is the Channel Assignment Method of Automatic, Freeze, or Off.

Automatic will run the DCA algorithm at the define Interval which is every 10 minutes by default. The anchor point is used to set the time of day the DCA algorithm would start.

For less frequent channel changes, increase the interval.

Keep in mind that clients will be disconnected briefly for a channel change.

Freeze does not run the DCA algorithm unless triggered.

Off disables the use of RRM and its auto-rf capabilities.

Avoid Foreign AP interference

Enabling this option allows RRM to take surrounding neighboring APs that are not part of your network into consideration.

RRM will create a channel plan to help mitigate co-channel interference.

Avoid Cisco AP Load

This option allows RRM to consider the traffic load on an access point to help build a better channel plan.

I don’t believe in enabling this feature because it can be difficult to make a client device associate with a specific AP.

Avoiding non-802.11a|b noise

This RRM option takes interference on the channel into consideration for the channel plan.

The category of interference is anything coming from a non-AP.

Persistent Device Avoidance

Sometimes there is non-Wi-Fi device interference affecting your wireless network. DCA can help mitigate the issue if Avoid Persistent Non-WiFi Interference option is enabled.

RRM will receive information from CleanAir and if the interference is creating a negative impact, RRM can issue a channel change to avoid it.

Viewing persistent devices can be done by viewing the details of an AP from the web interface. At the bottom of the page are all the persistent devices being tracked including the class type, on what channel the interference was heard, what the duty cycle is, RSSI, and when it was last seen.

Persistent Device Avoidance

DCA Channel Sensitivity

DCA has three different modes it could be in:

  • Scheduled – DCA algorithm runs at a selected time. Usually to minimize changes to off peak hours.
  • Steady State – DCA algorithm running at a specified interval.
  • Startup Mode – Used when making changes to the wireless network architecture. It restarts the DCA algorithm and runs for 100 minutes at high sensitivity.

By default, DCA has a medium channel sensitivity at 15 dB for 5 GHz and 10 dB for 2.4 GHz. This is the DCA Sensitivity Threshold.

By changing the channel sensitivity to high, then a cost metric of 5 dB better will recommend a channel change.

DCA Channel List

To have DCA dynamically select a channel, it must be in the Channel List. You can select specific channels for DCA to select from or select them all, including the UNII-2 channels.

Dynamic Bandwidth Selection

RRM has a flexible way to assign bandwidth to APs by analyzing the RF and selecting the best channel width.

By default, DCA uses 20 MHz channels. You can configure Cisco RRM DCA all the way up to 160 MHz channels or select Best to have RRM pick for you.

If an AP doesn’t support 40/80/160 MHz channels it will not be configured for it.

Dynamic Bandwidth Selection

To see DCA dynamically select channels, head over to the CLI on the controller and enter the debug command:

debug airewave-director channel enable

*RRM-CLNT-5_0: Aug 27 12:04:48.270: 58:bc:27:33:33:33 Slot 1: Channel set request on 58:bc:27:33:33:33(1) to (100, 104, 0, 0) 40 MHz Before -62, After -72  Second best: ch 157 wd 40 mt -86 RRF:enabled(0) metric(199) 
*RRM-CLNT-5_0: Aug 27 12:04:48.270:    [spamGetMeshBhRRMStatus] 0 
*RRM-CLNT-5_0: Aug 27 12:04:48.270: 58:bc:27:33:33:33 new channel width label set from 3 to 4 
*RRM-CLNT-5_0: Aug 27 12:04:48.270: 58:bc:27:33:33:33 slot 1 Set new channel 100 width label 4 
*RRM-CLNT-5_0: Aug 27 12:04:48.270: 58:bc:27:33:33:33 sent channel to spam  and call trap(0) 
*RRM-CLNT-5_0: Aug 27 12:04:48.270: 58:bc:27:33:33:33 Airewave Director: Final Channel Assignment (100,104,0,0) configured on AP  58:bc:27:33:33:33(1) 
*RRM-CLNT-5_0: Aug 27 12:04:48.270: 58:bc:27:33:33:33 Airewave Director: set channel on cell 58:bc:27:33:33:33(1) to 100 from 100 to reduce co-channel interference from (-128,-128,-62) to (-128,-128,-72) reason 1

In the output above, Cisco RRM DCA made a request to change an AP to a 40 MHz channel width using primary channel 100 and secondary 104.

At the very end, RRM sets the channel to 100 with the reason – to reduce co-channel interference.

From command line, it is possible to view when the last channel assignment was set using the following show command:

show ap auto-rf 802.11a <ap-name>

  Channel Assignment Information 
    Current Channel Average Energy...............  -71 dBm 
    Previous Channel Average Energy..............  -71 dBm 
    Channel Change Count......................... 541 
    Last Channel Change Time..................... Sat Aug 27 12:14:59 2016 
    Recommended Best Channel..................... 116

802.11a – shows the 5 GHz radio
802.11b – shows the 2.4 GHz radio
<ap-name> – replace with the name of your AP

The output was truncated just to display the channel assignment information. You can see the current and previous channel average energy, how many times the channel has changed, when it was last changed, and which is the best recommended channel.

Event Driven RRM

If you need the wireless network to react quickly and automatically to bad interference, ED-RRM is an option to enable.

An example could be a device using 100% duty cycle causing all nearby APs and clients to hold off communicating on the wireless medium. ED-RRM would allow an AP to make the channel change sooner than when the DCA interval kicks in.

To configure ED-RRM:

1. Click on Wireless
2. Click on 802.11a/n/ac or 802.11b/g/n
3. Click on DCA
4. At the bottom of the page, enable EDRRM
5. Set the sensitivity threshold to Low (my recommendation is not go above Medium)

Event Driven RRM

The sensitivity threshold defines the air quality (AQ) at the following:

  • Low sensitivity – AQ at 35%
  • Medium sensitivity – AQ at 50%
  • High sensitivity – AQ at 60%

An air quality of 100% is good, very bad is 0%

The Rogue Duty-Cycle option will allow you to set the percentage of duty cycle and rogue duty cycle occurs before triggering EDRRM.

To learn more details about Cisco’s RRM – DCA be sure to check out the white paper from Cisco.

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About the Author
Rowell, CWNE #210, is a network engineer in Higher-Ed. He enjoys working with wireless networking technologies and loves to share and engage with the community. You can connect with him on Twitter, LinkedIn, and Facebook.

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