Seamlessly Authenticate with a Secondary Active Directory Server (when primary is down)

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http://serverfault.com – How do you get workstations to (seamlessly) authenticate with a secondary Active Directory server when the primary one is down? Background: I added a secondary Active Directory server to a company's network, hoping that it would do authentication in the event that the primary Active Directory server was down. Although, the Secondary Active Directory server seems to be replicating correctly, authentication doesn't occur while rebooting the primary Active Directory server. Do I have a misunderstanding regarding the role of a secondary Active Directory server, or are there additional settings (HowTos)