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I have domain controller installed in a network and another server which acts as a backup DC. When the primary DC fails, How does other objects in the network know to connect to backup DC to resolve DNS and use it for authentication purposes. IS there a way to implement automatic failover for primary DC?

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up vote 4 down vote accepted

Active Directory uses DNS SRV records to locate domain controllers. Unless you've taken the time to set up a different AD Site where that other DC lives, chances are your clients are already using that other domain controller. When one DC is down, there will be some timeouts as clients attempt to talk to the dead DC but will automatically connect to any other DC in the domain.

Active Directory is a multi-master database. Unlike Windows NT, the concept of Primary and Backup DCs is no longer valid. All domain controllers can serve all authentication requests at all times. DNS is used to list the valid DCs, and clients pick the DC to talk to from that list.

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So do I need to enter IP of backup DC as an alternate DNS in every object in the network? – kishore Jan 3 '11 at 17:06
If your DC is also your DNS server, then you'll need to take steps. You'll need to set up the other DC as a DNS server for that to work (happily this is very easy). And then for every domained client, add a Secondary DNS server with the IP of your other DC. AD and dynamic DNS handle the rest of it. – sysadmin1138 Jan 3 '11 at 17:40

What you really have is 2 Domain Controllers installed - the concept of Primary and Backup Domain Controllers died long ago. They are both acting as Domain Controllers without any intervention from you.

In short, you've already done what you intended by running dcpromo on the subsequent Domain Controller.

This article describes how it works in Windows XP - I don't think the process has changed that much with later Operating Systems.

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