Creating an Additional Domain controller
This is same as creating a new DC for a new forest except small change where we have to select new DC to an existing the forest.
Goto “run” -> type “dcpromo.exe” click Next.
Click Next

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Click Next
For a new DC in a new forest select “ Create a new domain in a new forest “ and click “next”.
Provide network credentials(for example “hyaking.com”) click Next.

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Select DNS server as additional option for this server.

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Provide location for database, log files & SYSVOL folders on local server and click Next.

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Authenticate by providing the password and click Next.
Check the summary of the procedure and check the details and click Next to continue.

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This will continue the installation of Active Directory service and reboot the server on completion.

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This server acts as a additional Domain controller for any Local network if Primary Domain Controller is at other location provided there should be a Site-to-Site VPN tunnel established between two locations.