Controller Based WLANs

How does AP load balancing work on Aruba APs in ArubaOS 3.x?

by on ‎07-01-2014 01:52 PM

Product and Software: This article applies to all Aruba controllers and ArubaOS 3.3.1.

 

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 and you have other APs in the same area covering the same location on different channels and with different signal RSSIs.

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

 

To use AP load balancing, follow these steps:

 

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:

 

Using the WebGUI:

 

Navigate to Configuration > AP Configuration > <AP Group Name> > Wireless LAN > Virtual AP > SSID Profile > Advanced tab.

 

or

 

Navigate to Configuration > Advanced Services > All Profiles > Wireless LAN > SSID Profile > <SSID Profile name> > Advanced tab.

 

1582 image 1

 

Using the CLI:

 

configure terminal

 

wlan ssid-profile "loadbalance"


no local-probe-response

 

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 it fails to connect many times. Update your wireless card driver regularly to have the latest features and bug fixes.

 

Parameters to Decide Before Configuring Load Balancing

 

  • 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 per AP-Group in ArubaOS 3.x.

 

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

 1582 image 2

 

Configuring the AP Load Balancing Feature in ArubaOS 3.3.1

 

Using the WebGUI:

Navigate to Configuration > AP Configuration > <AP Group Name> > RF Management > RF Optimization Profile.

 

or

 

Navigate to Configuration > Advanced Services > All Profiles > RF Management > RF Optimization Profile > <Profile name>.

 

1582 image 3

 

Using the CLI:

 

rf optimization-profile "loadbalance"
ap-load-balancing
ap-lb-user-high-wm 20
ap-lb-user-low-wm 15
ap-lb-util-high-wm 90
ap-lb-util-low-wm 80

 

 

 

Best Practices

 

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

 

  • Enable load balancing if you need this feature.

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

  • Keep a difference of 3 to 5 between user count high and low water marks.

 

 

Monitoring AP Load Balancing

 

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

(Aruba)# show log wireless x

Jun 1 13:12:40 :404041: <WARN> |AP TEST@192.168.0.234 sapd| AM 00:0b:86:31:8e:a0: Enabling Load Balancing on AP 00:0b:86:31:8e:a1 NU <high-water-mark or higher> (if it is number of users based)

Jun 1 13:12:40 :404041: <WARN> |AP TEST@192.168.0.234 sapd| AM 00:0b:86:31:8e:a0: Enabling Load Balancing on AP 00:0b:86:31:8e:a1 BWR <high-water-mark or higher> (if it is utilization based)

 

 

If you then issue the 'show ap monitor ap-list' command, you will see that load balancing is enabled for all APs that reached the HWM and is disabled for all others.

(Aruba) #show ap monitor ap-list ap-name RAP2

Monitored AP Table
------------------
bssid                        essid      chan   ap-type phy-type   dos        mt   it load-balance nstas avg-rssi curr-rssi wired-macs
-----             -----            ---- ------- -------- ---     --   -- ------------ ----- -------- --------- ----------
00:0b:86:31:8e:a1    test          6         valid  80211b/g disable 1527 0  enable         2      15       15        3

 

 

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

 

Jun 1 13:34:19 :501089: <NOTI> |stm| Probe request: 00:19:7d:1e:8a:b7: Dropped AP 192.168.0.234-00:0b:86:31:8e:a1-TEST for Load Balancing SSID test
Jun 1 13:34:19 :501109: <NOTI> |stm| Auth request: 00:19:7d:1e:8a:b7: AP 192.168.0.234-00:0b:86:31:8e:a1-RAP2 auth_alg 0
Jun 1 13:34:19 :501094: <NOTI> |stm| Auth failure: 00:19:7d:1e:8a:b7: AP 192.168.0.234-00:0b:86:31:8e:a1-TEST Reason AP is resource constrained

 

 

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

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

 

(Aruba)# show log wireless all

Jun 1 13:38:09 :404039: <WARN> |AP TEST@192.168.0.234 sapd| AM 00:0b:86:31:8e:a0: Disabling Load Balancing on AP 00:0b:86:31:8e:a1 BWR X NU <low-water-mark or less> (if it is number of users based)

 

Jun 1 13:38:09 :404039: <WARN> |AP TEST@192.168.0.234 sapd| AM 00:0b:86:31:8e:a0: Disabling Load Balancing on AP 00:0b:86:31:8e:a1 BWR <low-water-mark or less> NU Y (if it is utilization based

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