WLC - Aggressive Load Balancing?

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Oct 15th, 2008
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Hello,


The Wirless LAN Network bulit is as follows -


1. 1 x 4404 WLC

2. 40 x LWAPP 1131AG Access Points

3. Windows Clients used by the Laptop Clients.

4. Only one Wireless VLAN across the Capmus network - hence AP's, WLC & Clients are all in one VLAN / IP Subnet.

5. No Access Point Group is created.

6. Aggressive Load Balancing is enabled allowing 15 Clients as max connection per Access Point.


Problem facing -


1. Tried configuring the Aggressive Loadbalancing allowing only 2 x Clients per AP. But noticed that the 3rd Client connecting to the same AP as of the previous 2 Clients have connected. 3rd client is not associating to a different AP which is nearby.


Please can one help me, if i'm configuring & testing Aggressive Load Balancing in the right way!


Regards,

Keshava Raju


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Overall Rating: 5 (4 ratings)
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Georgios Nikitas Thu, 10/16/2008 - 05:55
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I think what Aggresive Loadbalancing does is to reject the first connection attempt a client makes(when it reaches the threshold) in hope that the client will try to connect to another AP. If the client persists to connect to the same AP then it is accepted to the AP. So it is not guaranteed that Aggressive load balancing will force the client to move to an AP. It depends on the software(of firmware) that the client card uses for determining in which AP to connect.


I hope I helped!

Scott Fella Thu, 10/16/2008 - 10:36
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Also, you can't set the number of users per ap, that is a threshold number for traps. It also depends on the clients to look at the beacons and decide on an ap. If you have older clients, they might not work well with aggressiv load balancing. To be honest, I havent used that feature in a long time and have never ran into issues.

amr.momtaz Sun, 03/28/2010 - 10:26
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An explaination to your problem could be the following


Aggressive load−balancing works at the association phase. If enabled and the conditions to load−balance are
met, when a wireless client attempts to associate to a LAP, association response frames are sent to the client
with an 802.11 response packet that includes status code 17. This code indicates that the AP is too busy to
accept any more associations.


It is the responsibility of the client to honor, process or discard that association response frame with reason
code 17. Some clients ignore it, even though it is part of the 802.11 specification. The standard dictates that
the client driver must look for another AP to connect to since it receives a "busy" message from the first AP it
tries. Many clients do not do this and send the association request again. The client in question is allowed on
to the wireless network upon subsequent attempts to associate.


The controller only sends one association response frame with reason code 17 to the client. If the client
decides to discard the reason code 17, the client can try the same AP again and this time the AP allows the
client to complete the association.


If the client honors the association response status code 17, the client then attempts to associate to a different
AP. For example, if load−balancing is enabled and the load−balancing window is configured as five clients,
when a sixth client tries to associate to the AP, the client receives an 802.11 Association Response frame with
status code 17, which indicates that the AP is busy.


Original document could be found at "http://www.ciscosistemas.com/application/pdf/paws/107457/load_balancing_wlc.pdf"

George Stefanick Sun, 03/28/2010 - 14:09
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AMR is on target. In fact I just completed 20 hours worth of testing with variuos clients with ALB for a white paper I am doing. Code 17 isnt honored by most clients and is only sent 1 time from the AP. The clients will contiue to attempt to associate to the AP and the AP will allow them on.


Here is a peek of my white paper "still in draft"


WLC - Cisco WLC Aggressive Load Balancing; What is it and where did it go in 6.0!

I've spent the majority of my WLC experience at code level 4.2. Not by choice really, more

based on the fact that 4.2 is pretty darn stable and it is the only safe harbor to date for the Cisco WLC. Healthcare and Enterprise enviroments are typically slow to move on upgrades, especially when things are operating fine. 

Since my latest project involves the deployment of hundreds of Cisco 1142s @ location grade, it required that I move to later code to support the 1142 access points. After much research, conversations with our

local Cisco Wireless SE, conversation with peers at other healthcare organizations, and direct contact with the aware team I had decided that 6.0.188.0 was a release that was of great interest.

As I start to get fimilar with the new code I am starting to see that things got moved around a little. One of the items is Aggressive Load Balancing. If you aren't fimilar with Aggressive Load Balancing (ALB) you definitly need to be and let me share why.

First lets look at what ALB is and how it works and then we will dive into the differences between the 4.2 code and the new options 6.0 gives us. ALB when enabled, allows the Cisco WLC to load balance wireless clients on access points that are joined to the same controller. “Key word here – same controller”. You can configure the load balancing window globally in the controller. What is the load balancing window you ask? Well is the maximum number of clients that should be allowed on the access point BEFORE it will start to load balance.

Lets assume for a moment you have an access point with 5 clients already attached. When client #6 sends association request to the access point the access point will kindly respond with an associaton response frame with the reason code of 17. The wireless client will see reason code 17 in the association response and will kindly find other access points to associate with. However, some devices will ignore this frame and yet still continue to try and associate to the access point. Note: The Cisco WLC will ONLY send 1 reassociation frame with a reason code of 17. It doesn’t flood the medium / client with multiple frames.

Its up to the client to honor this information and move on. But I can tell you from my experience and testing this isn’t always the case.

By default, 4.2 and 6.x both have a load balancing window of (5). Lets look at an example.

The window setting controls when aggressive load−balancing starts. With a window setting of five, for

example, all clients after the sixth client are load−balanced.

I know, what is the reason code talk, right. Lets cover this as well. If you dive into the 802.11 frames you will see “Reason Codes”. When a client sees the reason code of “17”, it indicates to the client that the access point is busy and the client should look else where.

yada yada yada

I will post the complete paper on my site: my80211.com in the next week or so ...







Rik19972000 Thu, 12/09/2010 - 05:57
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I was reading about ALB and had 3 small questions


question1

------------------

Cisco doc says:


ALB does not take place if the threshold load-balancing window is not exceeded. That is, if the AP requested by the client is a valid candidate AP (current user count <= window + min users).


What is the value of min users? window is a value to be configured.


question 2

-----------------

Is ALB required to be enabled globally before you can enable/disable it on wlans?


From what I've tested with version 6.0.196: When it's disabled globally (wireless>advanced, uncheck the box) and leaving it enabled under wlan>wlan id> advanced) then there is no trace in the configuration of loadbalance wlan.


Also 'show load-balancing'  gives me:


show>load-balancing

Aggressive Load Balancing........................ Disabled
Aggressive Load Balancing Window................. 5 clients
Aggressive Load Balancing Denial Count........... 3

                                                    Statistics
Total Denied Count............................... 0 clients
Total Denial Sent................................ 0 messages
Exceeded Denial Max Limit Count.................. 0 times
None 5G Candidate Count.......................... 0 times
None 2.4G Candidate Count........................ 0 times



So it's required to enable it globally before you can enable/disable it per wlan.


Question 3

---------------------


I've on this forum that the ap will send only once the status code 17 busy, so the client associates on second attempt, no matter of what is configured for client denial count?

lorenzopaca Sun, 12/12/2010 - 12:15
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Hello,


How can we to know if clients have understood the code "17" ?


Is it a special NIC Cisco or is it available for CCX extension (and what version) ?


Thank you.


lorenzo

Paul Davidson Tue, 02/17/2015 - 09:10
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(1) I presume, AP Association verification is the best way to check the feature is working on the WLC in the GUI or at CLI:

>show load-balancing status,

furthermore through packet interception using Wireshark, Omnipeek or other.

(2) Lots of references on the web refer to Code(17) ratification in the IEEE 802.11-2007 standard, but this costs $300-500 to download in pdf from the IEEE! Try though I might, I cannot find any references to CCX regarding Code(17). I presume this is because it is an IEEE standard not proprietary to Cisco. Any Wi-Fi Alliance NIC certified post 2007 should be compatible with Code(17) or indeed any WNIC post 2007 firmware update enabled on the client.

On the WLC, Cisco AireO/S v4 onwards supports it for sure (server side). I run v4 on my 2006 WLC. I can vouch for AgLB on v4 even if it's just either on/off, 0-20. v4 is after all legacy.

Please rate if you find this useful. Thanks

 

 

Paul Davidson Tue, 02/17/2015 - 06:50
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 Cisco WLC v6 documention at:

https://supportforums.cisco.com/document/81896/load-balancing-and-band-select-cisco-wireless-lan-controller#Aggressive_Load_Balancing_Legacy

...states, " Client Window size is interpreted as the minimum number of clients that should be on the AP before the load-balancing mechanism kicks in. The default value is 5." It goes on to say the 'Client Window' setting allows for a minimum of 1, maximum 20.

However my V4 AireO/S states at the CLI, that the 'Client Window' input arguments are 0-20! I presume 0 is no preference, 1 is one a crowd etc. It's basic AgLB enabled or disable).

However yet again and in contrast, yet another Cisco AireO/S v6 document gives further argument (options are always good, Cisco excels in this, they give admins true control), at:

http://www.cisco.com/c/en/us/support/docs/wireless/4400-series-wireless-lan-controllers/113160-aggressive-load-balancing-clients-00.html

....applying to the 4400 series WLC AireO/S (v6 again) elaborates (by using the comparatives 'least' & 'lightest' & 'lowest'):

"Client Window Size - The load-balancing window and the number of client associations on the AP with the lightest load, determine the load-balancing threshold. You can enter a value between 1 and 20 for this parameter.

  • load-balancing threshold = load-balancing window + client associations on AP with lightest load

APs with more client associations than this threshold is considered busy, and clients can associate only to APs with client counts lower than the threshold.

Looking at our example setup, LAP 2 has the least number of clients (6). If the window size is set at 5, then every AP can have 11 clients (6 + 5 = 11). As a result, every 12th client is load balanced. If any client tries to join LAP 1, a denial response is sent from LAP 1. For a client, the denial message is sent based on the configured value for the maximum denial count.

Maximum Denial Count - The Maximum Denial Count parameter allows the user to configure the number of times the client associations will be rejected for a particular AP. The Maximum Denial Count can have a value between 0 and 10.

Note: The controller only sends an association response frame with reason code 17 to the client based on the denial count number. If the client decides to discard the reason code 17, the client can try the same AP again. This time, the AP allows the client to complete the association if it has reached the denial count already. If the client honors the association response status code 17, the client then attempts to associate to a different AP.

Once this is configured, you can enable or disable load balancing on a particular WLAN. In order to do this, choose WLANs from the controller GUI, choose the desired WLAN, and click the Advanced tab. Select the Load Balancing radio button as shown here:"

So to recap, from what I understand, (1) the minimum number of Clients for AgLB is 1 (or zero depending on (legacy) firmware version) let's not split hairs), BUT importantly, this is RELATIVE TO OTHER WLCS IN THE RF GROUP! In other words, 1 more...two more, than it's peers etc.(3)The AgLB Denial Count is the number of knock backs a client will be sent. Remember it can proceed anyway, despite 'if' the max knock Denial Count is reached, & (2), despite that which in my experience is the default for Cisco, that LOCAL settings over-ride GOBAL, the latter document states that, "Once this is configured (by this I presume AgLB global setting), you can enable or disable load balancing on a particular WLAN". Thus GLOBAL AgLB must be configured before granular control of LOCAL AgLB per WLAN can be tuned.

If anyone can shed further light or interprets this differently please let me know.

Please rate 5* if you found this useful. ;-) thanks

George Stefanick Tue, 02/17/2015 - 09:42
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Good read Paul +5

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