Lowering Active Directory Domain / Forest Functional Level

Well… my previous post about my sole domain controller corrupting caused me to go off and promote a new domain controller as a contingency.

When I set up my lab, I set the functional level to Server 2012 in order to see what the differences would be. My current domain controller is Server 2012, but my other hosts are all 2008 R2… because of this, I couldn’t promote my existing hosts to domain controllers & didn’t fancy running up a new 2012 box purely for this backup role. Running dcpromo on the host in it’s current state results in an error “The functional level of the domain or the forest is incompatible with this operating system”

So it left me with the question, how am I going to rollback the functional level to 2008 R2 to let me do this? Well, PowerShell obviously…

Firstly I had to lower the forest to the required level, via:
Set-ADForestMode -Identity domain.local -ForestMode Windows2008R2Forest
(substituting domain.local for the FQDN of my domain)

And then I could lower the domain functional level:
Set-ADDomainMode -Identity domain.local -DomainMode Windows2008R2Domain

Then running a dcpromo on my 2008 R2 host again allowed me to configure it as a new domain controller.


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