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As I plan upcoming AD upgrade (from 2008R2 to 2019) I face another problem: we're actually have AD Forest that consists of several sites, each of them on 2008R2 AD version. As we upgrade one site AD version from 2008R2 to 2019, other sites will remain on 2008R2 for quite site time until we'll see 2019 scheme works well and no problem appears. And now I doubt if the whole forest be functional and no problem arise when one site be on 2019 and other sites on 2008R2 AD scheme versions.

So the question is: in practice (that is, in real life) will the forest still be functional and usable as one site (and, eventually, one by one all other sites, too) be upgraded, while remaining sites still be on lower AD version? In general, the forest needed to have cross-sites auth, so no sophisticated features are needed, but anyway I'd better ask before jumping into the water :)

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When you say sites I assume you mean locations, and not domains... meaning you have different locations in a single domain that have Domain Controllers running different Windows Server versions, and not that you have different Domains in the same Forest.

If it is the case that you have different sites in the same Domain that have Domain Controllers running different Windows Server versions then you can add new Domain Controllers as long as they are supported in your current Domain Functional Level and Forest Functional Level. All Domain Controllers will operate at one DFL and FFL regardless of their Windows Server version.

Long story short, can you have Windows Server 2019 Domain Controllers running with older Windows Server version Domain Controllers? Yes, so long as you meet the requirements..

https://docs.microsoft.com/en-us/windows-server/identity/ad-ds/active-directory-functional-levels

  • In fact, we use different subdomains as well (like branch1.company.com at first branch office, branch2.company.com at second etc.) These all connect under the forest. Will it change the picture in the case of AD upgrade? – Alexander May 7 at 1:13
  • No. You just can't raise the Forest Functional Level until all Domains are at that same level, just the same as you can't raise the Domain Functional Level until all the DC's in that Domain are at that same level. Eg. if you have two DC's in branch2.company.com, you must first replace both with Servers 2019 to raise Domain Functional Level to Server 2016 (if I remember correctly there is no level Server 2019). Then you must raise all Domains to level Server 2016 and at the end raise the Forest Functional Level to Server 2016. Just don't forget to do the adprep before you start with anything. – toofat May 16 at 19:52
  • Ahhh yes, and read very carefully that document "joeqwerty" posted. I've migrated all my DC's from FRS to DFS ages ago. If your Domain started at level 2000 (like mine) or 2003, you will have to migrate to DFS first (if you haven't done that already). – toofat May 16 at 20:00

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