Before add a domain controller that runs Windows Server 2012 to an existing Active Directory forest, the forest functional level must be Windows Server 2003 or higher. In the installation process, ADPREP has been integrated into the AD DS installation and runs automatically as needed.
Let’s install the Active Directory Domain Services Role.
Click Add roles and features
Select Role-based or feature-based installation.
Select the Server.
Select Active Directory Domain Services.
Click on Promote this server to a domain controller.
Or you can click the same link in the status icon also.
Select Add a domain controller to an existing domain. Specify the domain information as well.
Specify domain controller capabilities and site information.
Specify additional replication options.
Specify the location of the AD DS Database, log files and Sysvol.
Setup is going to prepare existing Forest, Schema and Domain.
Server will automatically restart after the installation is done.