The suggestion to use preferred domain controllers is just to ensure your vserver doesn't attempt to contact the failed domain controller. EG
cluster1> active-directory preferred-dc add -vserver vserver1 -domain testlab.local -preferred-dc 192.168.100.10
To resolve your issues, first i would recommend you perform a metadata clean in Active Directory to remove the failed domain controller (if you haven't already done so):
Then you can run the following command to rediscovered the domain controllers in the AD site for the vserver:
cluster1> active-directory discovered-servers reset-servers -vserver vserver1
Hope this helps?
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