Hi Friend,

When APs join controller you can define primary controller and secondary controller for each specific APs so that if by any chance any controller is unreachable APs can join second controller with minimal downtime.

Also you can configure mobility group between 2 controllers so that when client roams to different APs on different controller they can retain thir ip address.

To read more about mobility solution read this link

http://www.cisco.com/univercd/cc/td/doc/product/wireless/control/c44/ccfig41/c41mobil.htm

HTH

Ankur

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