Hello 金敏,
Thank you for posting in our Q&A forum.
Q:When one AD breaks down, how does the client that has logged in automatically switch to the other AD for verification? What is the process?
A: Yes, if there are multiple DC available it the same site as this client, when one DC in this site is down, the client that has logged in automatically switch to the other AD.
For more information, please refer to link below.
https://social.technet.microsoft.com/wiki/contents/articles/24457.how-domain-controllers-are-located-in-windows.aspx
Hope the information above is helpful.
If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou
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