Deploying Read Only Domain Controllers
You can install an RODC on a full installation of Windows Server 2008 or on a Server Core installation of Windows Server 2008.Before you do, there are few things to check first. Ensure that the forest functional level is set to Windows Server 2003 or higher. Run adprep /rodcprep (You can skip this if you are creating a new forest that will have only domain controllers running Windows Server 2008.) To run adprep /rodcprep, you must be a member of the Enterprise Admins group.
An RODC must replicate domain updates from a writable domain controller that runs Windows Server 2008. Before you install an RODC, be sure to install a writable domain controller that runs Windows Server 2008 in the same domain.
The domain controller can run either a full installation or a Server Core installation of Windows Server 2008.
To install an RODC on a full installation of Windows Server 2008, you must be a member of the Domain Admins group.