1. When AP disassociate to the WLC, the AP will divert to standalone mode aotomatically, right ?
Yes
2.when AP in standalone mode , the AP will authenticate the client by itself, right ?
Only if your wlans is configured with "Local Authentication"
3. In standalone mode, AP store authentication information in nonvolatile memory, is that means no radius server should be used for authentication ,only PSK can be used ?How about a new client want to connect a standalone AP ? What is the username and password should be used for this new client ?(assume this is the first time the client connect to this ap. )
No, 802.1X also supported. In standalone mode, AP itself act as Authenticator & communicate with RADIUS server (it is assume RADIUS server still available even WLC is down). Refer this design guide
http://www.cisco.com/c/en/us/td/docs/wireless/controller/8-1/Enterprise-Mobility-8-1-Design-Guide/Enterprise_Mobility_8-1_Deployment_Guide/ch7_HREA.html#57480
4.In standalone mode ,We should deploy local DHCP server for client , right? How to avoid IP overlapping ? required an unique IP address space for this ?
Typically in FlexConnect, you terminate traffic locally in branch switches. So ideal to have DHCP service locally in branch.
HTH
Rasika
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