Edward,

There is noting like windows database failover but you can select other database or domain if user not found or typed invalid username/password in windows database.

Unknown User Policy—If the unknown user policy contains additional external databases and the Windows database is not the last database on the Selected Databases list, you might enable this option. For example, If a user does not exist in the Windows database, or has typed an incorrect password, the error 1326(bad username or password) is returned. ACS treats this error as a wrong password error and does not default to another external database. You should enable this option when additional external databases appear after the Windows database in the Selected Databases list. When enabled, ACS searches for the unknown user in the other external databases.

Configure Domain List—ACS tries to authenticate to any domain listed in Available Domains. If your Windows users do not specify their domain when dialing up, ACS relies on Windows to try to locate the appropriate user account. However, Windows may not be able to authenticate a user properly if the same username exists in more than one trusted domain. We recommend that you ask users to enter their domains when dialing in. If this is not practical, you can define a Domain List. If ACS fails to authenticate a user because the account exists in more than one domain and a Domain List exists, ACS will then retry authentication for each domain in the list. The list order is significant: domains that appear earlier in the list will be tried first. Because of the delay (typically two seconds) for each domain that fails authentication, you should set your AAA client timeout accordingly.

For more information, please visit the below listed link:

http://www.cisco.com/en/US/docs/net_mgmt/cisco_secure_access_control_server_for_windows/4.1/user/UsrDb.html#wp354338

Regds,

JK

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