Below I'm going to go into the prerequisites for a Read Only Domain Controller:
- The PDC Emulator must be on a Windows Server 2008 Domain Controller
- Must recieve updates from a Windows Server 2008 DC. This means your AD site link with the lowest cost in which the RODC will recieve updates from must have a Windows Server 2008 DC.
- DFL/FFL must be at least Windows Server 2003 functional level.
- Must have run ADPREP /RODCPREP on your domain
- Only one RODC Per Domain, Per Site. In an active directory site you can have only one RODC, however if you have multiple domains you in a forest it breaks this rule as you can have one in each site per domain.