1. Verify that all DCs in the domain are, at a minimum, at the OS version to which you will raise the functional level. Yes… I know this sounds obvious, but you’d be surprised. What about that DC that you decommissioned but for which you failed to perform metadata cleanup? Yes, this does happen.
Another good one that is not so obvious is the Lost and Found container in the Configuration container. Is there an NTDS Settings object in there for some downlevel DC? If so, that will block raising the Domain Functional Level, so you’d better clean that up.
2. Verify that Active Directory is replicating properly to all DCs. The Domain and Forest Functional Levels are essentially just attributes in Active Directory. The Domain Functional Level for all domains must be properly replicated before you’ll be able to raise the Forest Functional level. This practice also addresses the question of how long one should wait to raise the Forest Functional Level after you’ve raised the Domain Functional Level for all the domains in the forest. Well…what is your end-to-end replication latency? How long does it take a change to replicate to all the DCs in the forest? Well, there’s your answer.
Best practices are covered in the following article:
322692 How to raise Active Directory domain and forest functional levels
There, you’ll find some tools you can use to properly inventory your DCs, and validate your end-to-end replication.
Update : Woo, we found an app that breaks! It has a hotfix though (thanks Paolo!). Mkae sure you install this everywhere if you are using .Net 3.5 applications that implement the DomainMode enumeration function.
FIX: "The requested mode is invalid" error message when you run a managed application that uses the .NET Framework 3.5 SP1 or an earlier version to access a Windows Server 2008 R2 domain or forest
Jonathan “ Con-Function Junction ” Stephens
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