Configuring a Backup Domain Controller
1) Log into the server you will be setting up as your backup domain controller (BDC).
2) Go to Start, Run and type “dcpromo" and hit OK.
3) The Active Directory installation wizard will start. Click Next.
4) Click Next on the Operating System Compatibility page.
5) On the Domain Controller Type page, select "Additional domain controller for an existing domain" (Figure 1). Click Next.
6) Provide network credentials for your machine to join the domain. This will need to be someone with domain administrator privileges. Enter the username, password and domain and click Next (Figure 2).
7) On the Additional Domain Controller page, enter the name of the domain you are joining. Click Next.
8) Use the default settings for the database and log locations. Click Next.
9) Keep the default settings for the SysVol (Shared System Volume) and click Next.
10) Enter the same password used when setting up the domain for the restore mode administrator password. This password is used when restoring a domain from backup media. Click Next.
11) On the Summary page, click Next to start configuring the BDC.
After a few minutes, you should receive noticed that the operation was successful. Reboot the computer after the operation completes and you should be able to log into your new backup domain controller.