I have a single domain network with one PDC, one BDC and an exchange server. When the BDC is off-line, the exchange server reports event 5719 "no windows nt domain controller available", despite the fact that the PDC is still functioning, I have checked the wins 1c record for the domain - it is ok and lists both dc's. The pdc is allowing users to log on ok - it just seems that exchange has decided to authenticate against the bdc - how do I ensure that exchange will authenticate against EITHER the PDC or the BDC.