It would seem that I need to do an Authoritative Restore, with DC1
You can't right now since DC1 and DC3 are in unknown broken states. More info here.
https://learn.microsoft.com/en-us/troubleshoot/windows-server/networking/frs-event-log-error-codes
Nonauthoritative restore
Use a nonauthoritative restore to return a member back into service, saving as much state from that member and from the direct replication partner
in the direction that replication is not working.
Authoritative restore
The computer that is configured for the authoritative restore is configured to be authoritative for all the data that you want to replicate to replica set members
- So use nonauthoritative to fix a single member
- When you use authoritative you're declaring one member the authority that the others will all inherit from
Another possibly simpler option to fix a single bad actor is to demote it, then after reboot promo it again.
This tool may also be helpful
https://www.microsoft.com/en-us/download/details.aspx?id=30005
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