Two Exchange Servers and three Domain Controllers in the same AD site. All Domain Controllers are GCs.
Exchange is Exchange 2016 on Windows Server 2016. Domain Controllers are Windows Server 2019.
On Exchange01 everything works fine – using the Exchange Management Shell I can issue commands such as Get-Mailbox successfully, and Get-DomainController returns a list of all Domain Controllers.
Exchange02 gives "Could not find any available Global Catalog in forest".
On Exchange02 I can use regular PowerShell and issue commands such as Get-ADUser which confirms AD connectivity is OK for this server.
On Exchange02 I can do Get-ADForest | Select-Object -ExpandProperty GlobalCatalogs and it correctly lists all Global Catalogs.
The evidence indicates that this is localised to Exchange connectivity to AD on Exchange02.
This has only happened over the last day and there have been no changes to the environment.
So – how do I fix it?
Best Answer
Resolved - this was a DNS problem.
Specifically, in my DNS forward lookup zone for my domain, the Name Servers, _msdcs node, _sites node, etc contained references to old or invalid Domain Controllers.
These references included:
Attempts to connect to one of these Domain Controllers were causing Exchange02 to fail to find a GC.
Cleaning out the bad records, then rebooting the Exchange servers (I could have rebooted only Exchange02, but I decided to do both with Exchange01 used as a sanity check) resolved.
Because Exchange is a platform that you can't reboot too regularly (at least without it being service-affecting) I believe that this problem would have eventually manifested on Exchange01, and possibly even disappeared from Exchange02, given a sufficient number of reboots.