The following bullets list the items you should review and complete before installing RODCs:
. Active Directory running on Windows Server 2003 or Windows Server 2008 R2 must already exist in the environment.
. The Active Directory schema must support the Windows Server 2008 R2 server extensions.
. The forest and domain functional level must be running Windows Server 2003 or higher.
. At least one domain controller within the domain must be running Windows Server 2008 R2.
. The PDC Emulator FSMO role must be running Windows Server 2008 R2.
. A regular non-read-only (writable) domain controller must already exist within the Active Directory infrastructure.
. The RODC cannot be the first domain controller within the Active Directory infrastructure.
. If the DNS service will be configured on a Server Core installation, a non-read-only DNS server must be present within the domain.
Source of Information : Sams - Windows Server 2008 R2 Unleashed