Controller Based WLANs

How does AP load balancing work on Aruba APs in ArubaOS 2.5.x?

Aruba Employee

Product and Software: This article applies to all Aruba controllers and ArubaOS 2.5.x.

 

AP load balancing is a feature that allows the administrator to distribute wireless users between access points (APs) in the same area. This feature is used mainly in conference rooms and during events when you have large number of users near one AP, while you have another AP in the same area covering the same location on a different channel and with a different signal RSSI.

 

Using this feature, the administrator can avoid having one AP be congested while other APs in the same area are idle or underutilized.

 

To use the AP load balancing feature:

 

1) Have more than one AP covering the same area. (ARM ensures they are on different, non-overlapping channels.)

 

2) Disable local-probe response for APs where load balancing is needed, which ensures that probe requests are processed at the controller.

 

To disable local probe response:

 

(Aruba) #configure terminal

 

(Aruba) (config)#ap location 0.0.0

 

(Aruba) (config)#local-probe-response disable

 

3) Ensure that the user wireless card driver will roam to another AP if it fails to connect to the nearest AP. Some old drivers continue to try to connect to the same high RSSI AP even after failing to connect many times. Update your wireless card driver occasionally to have the latest features and bug fixes.

 

Parameters to Determine Before Configuring AP Load Balancing

The four major settings in load balancing are:

 

  • Low water mark: can be set for number of users and network utilization.

  • High water mark: can be set for number of users and network utilization.

  • Max Association Retries: the number of probes that the AP will ignore from the same user to force him to try another BSSID / AP on the same SSID. If the user driver continues to probe the same BSSID/AP, the AP responds after the "Max Retries" of probes. The AP finally responds to avoid disconnecting users with an old or bad driver. Setting this parameter to a very high number might disconnect this type of wireless user.

  • Wait time: time in seconds to wait before enabling or disabling load balancing after the threshold is reached.

High water mark is the threshold that enables the load-balancing state on the AP. In this state, the AP stops responding to probe requests from a wireless user. Normal drivers try another BSSID (another AP), so we can guarantee that the number of users will not exceed the high water mark on the AP.

 

When users begin to leave the AP, this AP stays in the load-balancing state until the number of user falls below the low water mark. In this case, the AP will leave the load-balancing state and start responding to probe requests from wireless clients (normal mode).

 

Note: These parameters are global in ArubaOS 2.5, so they will apply to all APs and SSIDs.

 

The following diagram shows how this feature works in conjunction with the number of users, high water mark (HWM), and low water mark (LWM).

 

1553_image001.jpg

 

Configuring the AP Load Balancing Feature in ArubaOS 2.5.x

 

Using the WebUI

 

Navigate to Configuration > Advanced > RF Management > Optimization > Load Balancing.

 

1553_image001.jpg

 

Using the CLI

 

configure terminal

 

wms ap-policy ap-load-balancing enable  >>>  to enable load balancing

 

ap-policy ap-lb-max-retries 8  >>>  number of retries, default is 8

 

ap-policy ap-lb-util-high-wm 90  >>>  utilization high water mark, default is 90

 

ap-policy ap-lb-util-low-wm 80  >>>  utilization low water mark, default is 80

 

ap-policy ap-lb-util-wait-time 30  >>>  wait time, default is 30

 

ap-policy ap-lb-user-high-wm 20  >>>  user count high water mark, default is 255

 

ap-policy ap-lb-user-low-wm 15  >>>  user count low water mark, default is 230

 

Best Practices

 

Change only the following parameters and keep all other values on the default:

 

  • Enable load balancing if you need this feature.

  • Change the user count low water mark and high water mark based on your requirements and the required bandwidth per user. Values from 15 - 25 for high and low water marks are usually used.

  • Keep a difference of 3 - 5 between user count high and low water mark.

 

Monitoring AP Load Balancing

When the threshold is reached (number of users or utilization on the radio), the following log message is generated:

Mar 6 13:09:00 am: AM 1.1.4-00:0b:86:40:85:80: Enabling Load Balancing on AP 00:0b:86:40:85:80 BWR X (if it is utilization based)

 

Mar 6 11:09:09 am: AM 1.1.4-00:0b:86:40:85:80: Enabling Load Balancing on AP 00:0b:86:40:85:80 NU X (if it is # of users based)

 

Now, if you issue the 'show am ap-list' command, you will see that load balancing is enabled for all APs that reached the high water mark. Load balancing is disabled for all other APs.

 

(Aruba) #show am ap-list 10.1.100.235

 

AP Table
--------
bssid essid chan ap-type phy-type dos mt it avg-rssi load-balance wired-macs
----- ----- ---- ------- -------- --- -- -- -------- ------------ ----------

00:0b:86:40:85:80 test 1 valid 80211g disable 928 0 50 enable

 

After load balancing is enabled for the AP (SSID), you will continue to receive the following log messages if you debug stm:

 

Mar 6 11:34:43 stm[504]: probe req Dropped AP 10.1.100.236-00:0b:86:40:83:c0-1.1.3 for Load Balancing SSID test

Mar 6 11:34:44 stm[504]: probe req Dropped AP 10.1.100.236-00:0b:86:40:83:c0-1.1.3 for Load Balancing SSID test

 

Also the 'show stm packets' command shows an "assoc-resp" with a resource-constrained error code.

 

After the low water mark is reached, the following log message is generated:

 

Mar 6 11:46:48 am: AM 1.1.4-00:0b:86:40:85:80: Disabling Load Balancing on AP 00:0b:86:40:85:80 BWR Y NU X

 

More Information

 

The KB article "How do I configure AP load balancing in ArubaOS 3.x?" is available on the Aruba support website and is Answer ID 638:

 

http://support.arubanetworks.com/Default.aspx?tabid=111&loc=https://kb.arubanetworks.com/app/answers/detail/a_id/638/kw/638

 

The KB article "How does AP load balancing work on Aruba APs in ArubaOS 3.3.1?" is available on the Aruba support website and is Answer ID 276:

 

http://support.arubanetworks.com/Default.aspx?tabid=111&loc=https://kb.arubanetworks.com/app/answers/detail/a_id/276/kw/276v

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‎07-02-2014 01:10 AM
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