Link : [1] http://technet.microsoft.com/en-us/library/cc772234(WS.10).aspx

This step-by-step guide provides instructions for planning, installing, and using a read-only domain controller (RODC).

Important:
Your system must meet the following prerequisites for deployment of an RODC:

• The forest functional level must be Windows Server 2003, so that linked-value replication is available. This provides a higher level of replication consistency. The domain functional level must be Windows Server 2003, so that Kerberos constrained delegation is available. Constrained delegation supports security calls that must be impersonated under the context of the caller.
• You must deploy at least one writable domain controller running Windows Server 2008 for the same domain as the RODC. This provides the RODC with a replication partner. To deploy a domain controller that runs Windows Server 2008, you must copy the contents of the \sources\adprep folder on the Windows Server 2008 installation DVD to the schema master and then run adprep /forestprep and adprep/domainprep /gpprep. For more information, see Prepare a Windows 2000 or Windows Server 2003 Forest Schema for a Domain Controller That Runs Windows Server 2008 at [2] http://go.microsoft.com/fwlink/?LinkId=109019.
• You must then run the adprep /rodcprep command before you install the first RODC. This step is not required if you are creating a new forest with only domain controllers that run Windows Server 2008. For more information, see Prepare a Forest for a Read-Only Domain Controller at [3] http://go.microsoft.com/fwlink/?LinkId=109020.