Authentication doesn't fail over to additional DC's

Hi,

I have 8 domain controllers in our enterprise. 6 are located at other
geographical sites and 2 are located in our central datacenter. All domain
controllers are handling logon requests through the enterprise. I assumed I
should be able to shut down one of the DC's in the datacenter without causing
logon issues, but that doesn't seem to be the case -- even if I transfer the
PDC emulator role to another DC. Users start calling the HelpDesk saying
they can't log on but more importantly, our BizTalk server won't authenticate
and all users are denied access to our intranet site which relies on BizTalk.
Once the DC controller is back on line, everything goes back to normal.
I've talked with the team controlling the BizTalk server and they assure me
that don't have any dependencies written into the server configuration
requiring that one DC to be online. I can shut down other DC's with no
interruption to authentication. Any ideas?
.

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