I am trying to understand how the load balancing threshold is calculated but I am finding conflicting information, even withing Cisco's own documentation. I would be grateful if anyone could help.
Cisco's latest Wireless LAN Controller Configuration Guide for software release 7.0.116.0 (April 2011) contains the following information for configuring Wireless > Advanced > Load Balancing Page (emphasis mine):
In the Client Window Size text box, enter a value between 1 and 20. The window size becomes part of the algorithm that determines whether an access point is too heavily loaded to accept more client associations:
load-balancing window + client associations on AP with highest load = load-balancing threshold
In the group of access points accessible to a client device, each access point has a different number of client associations. The access point with the lowest number of clients has the lightest load. The client window size plus the number of clients on the access point with the lightest load forms the threshold. Access points with more client associations than this threshold is considered busy, and clients can associate only to access points with client counts lower than the threshold.
Option 1
The formula shown is correct (load-balancing window + client associations on AP with highest load = load-balancing threshold). If so, this would mean that if you had a window size of 5 and the AP with the highest load at the time of calculation was 15, the threshold would be 18. However, as no APs have 18 associations then this threshold would never be reached. Even if an AP reach 18 associations, the next client trying to associate would trigger another calculation for the threshold which would be 21 (3 + 18) and so still, this threshold would never be hit.
Option 2
The description in the paragraph below is correct (The access point with the lowest number of clients has the lightest load. The client window size plus the number of clients on the access point with the lightest load forms the threshold). This sounds much more sensible to me. In this case, the window size was 3 and the AP with the lowest number of associations already had 7 clients associated, the load balancing threshold would be 10 i.e. no load balancing would occur until a client tried to associate with an AP which already had at least 10 clients associated.
Option 3
I have seen many descriptions on forums etc of the load balancing threshold being essentially the Client window size, i.e. if the client window size is 3 then load balancing will kick in when a client tries to associate to an AP with at least 3 clients already associated. This doesnt match the above documentation unless the AP with the least number of clients associated doesnt have any associated clients i.e. 0 clients.
Questions
- I think Option 2 is the correct description of load balancing and the formula given stating use of the AP with the highest load is a typo (albeit still not corrected in the latest documentation). Am I correct?
- The problem with using the option 2 method of calculating the load threshold is that you will be unnecessarily performing load balancing in an environment where some of your APs do actually have zero clients associated, unless you set the window size to somehing close to 10.
- I read here http://www.perihel.at/wlan/wlan-wlc.html#aggressive-load-balancing that when calculating the load threshold, it only accounts for the 8 'best' APs for a given client. In other words, if you have 60 APs on your campus but only 20 are visible to the client, the controller will only perform its load threshold calculations bases on the 8 APs which have the best signal to the client. This would ,ake sense as there is no point setting a load threshold based on the lightest loaded AP which is not even within 'reach' of the client. Is this correct as I can not find any other documentation which supports this?
Thanks in advance for your help with this.
