With a previous versions(2008r2,2008,2003) we have to manually run adprep and forest prep in some cases(Before you add the first domain controller that runs a version of Windows Server that is later than the latest version that is running in your existing domain,Before you upgrade an existing domain controller to a later version of Windows Server, if that domain controller will be the first domain controller in the domain or forest to run that version of Windows Server. ).But with server 2012 its inbuild process and we dont have to run any of these tools.
Server 2012 domain controller on windows 2003 domain
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