Hi Tony,
thanks for your rating and glad your problem was solved. I do not see any short document on this principle, but this is how it works:
Speeds can be mandatory, supported or disabled.
"Mandatory" means that the client HAS to be able to support that speed to be allowed to associate. "HAS" does not mean NOW, it's just a driver support.
"Supported" means that whether the client driver supports that speed or not, the client can associate.
"Disabled" means that this speed cannot be used on this AP.
That's the background. Usually, several speeds are "mandatory" by default. Each of them matches one modulation method (how you encode the frames you send). Making them mandatory ensures that all clients understand what the other clients are saying.
The AP tells its clients which speeds are mandatory, supported and disabled, and clients comply (if they can).
So one thing you are sure of, is that all clients support all mandatory speeds.
The issue is that clients close to the AP will use high speeds (mandatory or supported), clients far from the APs will revert to slower (but safer) speeds (mandatory or supported).
So when you want to send a message to all clients in the cell, you are going to use a mandatory speed (so that ALL clients understand it), and a low speed (so that far away and close clients can get it). You typically use the lowest mandatory speed.
When you want to send a multicast message, you think it as a type of broadcast, as you send it to more than one client. To be on the safe side, you would send it at the lowest mandatory speed.
The problem is that Cisco, when this feature started to be supported, sent it at the HIGHEST mandatory speed instead of the lowest. Why? Long story... but in any case, this "abnormality" is corrected in 6.0.
For all codes before 6.0, the trick I mentioned is the way to go around the problem.
Just a warning: the lowest mandatory speed is also the speed used to send beacons... so you probably want to set the lowest mandatory speed to the lowest speed you support, for example 6 Mbps. If you set your mandatory speed to 12 Mbps, but still allow 9 and 6 Mbps, you will have clients that will associate to your cell and that will not get beacons and multicasts when they move away from the AP (from the 12 Mbps zone to the 9 or 6 Mbps zone), and this will create problems. So you can disable 6 and 9 (and leave mandatory to 12), or push mandatory to 6 (and set 9 and 12 to supported). The second choice is usually better (because you may have wireless devices in your 6 Mbps area, trying to discover the network, and sending probe requests without realizing that they are colliding with your AP messages).
Hope it's clear enough!
:-)
