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I need to upgrade my AD DFL and FFL from Windows server 2003 to 2008R2. As the functional level downgrade is available to server 2008 only. Hence, I would like to know is there any way that I could downgrade it back to 2003 or any recovery plan if there's anything not working with 2008R2 functional level?
I believe a VM snapshot or backup will not work in this case since the AD database is replicated across all the DC. Please correct me if I'm wrong.
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One option may be to take a full backup of the PDC emulator.
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If restore the full backup of PDC to our infrastructure, what will happen for the remaining writable DC? Will there be any AD database consistency issue?
Generally speaking in a multi-dc environment if there were some catastrophic failure one would not restore multiple domain controllers. Restore one, seize roles (if necessary), do the metadata cleanup and rebuild the others from scratch.
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With this approach do I still need to do an authoritative recovery of the Sysvol?
I wouldn't think so.
Just checking if there's any progress or updates?
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The only way to downgrade from a FFL 2008 or 2008 R2 to 2003 is to do an Active Directory Forest Restore
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/manage/ad-forest-recovery-guide
And no, a snapshot will not work.
In this case I'll have to take either a full backup or system state backup and follow the steps on the link right. So in that case, do I need to shutdown all the DC, and then restore only one writable from the backup and proceed further? The backup has to be the PDC or any writable DC will do?
You could keep 1 DC offline in case. But the full backup of a DC that is also a Global Catalog is mandatory.
Personally, i have done several AD forest upgrades in my career and i never had to downgrade the AD Forest... but prevention is better than cure ;)
Before doing the upgrade in your production environment, i would suggest to practice your recovery procedure in an isolated environment. So if you have any issues, you will be prepared.