In my environment, I currently have three domain controllers. My main DC is running on 2012R2 and the other two are running on 2016. The current domain functional level is 2008R2 and the Functional level is 2003. I don't know why it's this way as I just found this out today.
I read that all DCs need to be on the same version of Windows server but obviously, this is not happening in my environment. Can I raise my DFL and FFL to Windows Server 2012R2 while my other two DCs are running on 2016 with no issues?