Well that changes thing... Your better off manually setting the channel then. Not really a good idea to have a mixed environment like that. Your roaming for sure will fail if users roam from one wireless to another.
It all depends on what you exactly mean by the word "fluctuation".
If you mean that the channel keeps changing when you choose automatic (each time a channel change happen the clients will disconnect) then that could probably be normal. why? well, suppose that you added the new wap200 that was heared by 3 1242 APs (3 different channels, 1, 6 and 11). In this case whatever the wap200 will choose as a channel then there will always be interference. depending on signal strength and distance between wap200 and the other APs, channel quality may change with time so the current good channel could be worse than another so the wap200 changes it is channel. It is also possible that 1242 APs change the channel if they detect the wap200.
The best thing to do is to to make good design so wap200 is being heard a minimum from other 1242 APs to avoid unwanted channel change. changing wap200 location may be considered to achive this goal.
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