Access points may deny client's association for various reasons. The first thing we need to do is to look at the management frames to understand why associations are dropped/denied. Here we will be discussing about a specific use case on why clients are dropped due to reason "AP is resource constrained-Max Clients Associated".
Whenever clients are failing to associate to an access point, we need to take a look at the management frames to understand the disconnect. Below command will list the management frames exchanged between the client and AP.
show ap debug mgmt-frames - This command will list management frames between IAP and all wireless clients trying to associate to the access point. show ap debug mgmt-frames <client mac> - This command will list management frames between IAP and the specified client trying to associate to the access point.
In the below example, the client was trying to connect to the access point but it's association was denied due to the reason "AP is resource constrained-Max Clients Associated". It means AP cannot accommodate any further association as it already reached it's hard limit.
The reason for denied association can also be seen from below command for specific client and additionally it provides details on max-client association on that radio(a/g).
The only way to fix this problem is to increase the number of association on the corresponding access so that access point can honor further associations. For example if the max association was set to 10 on "a" radio, WLAN administrator can increase the association count as needed based on the average number of clients connecting to that access point using below command.
Instant# a-max-clients 15 Instant# g-max-clients 25
Note: Value of max-client association must be in the range 1 to 255.