One of my earliest experiences with using DCPROMO to demote a domain controller, instead of its more common use to promote a member server to become a DC, was several years ago. I was teaching an intro Active Directory class for a company planning a migration to Windows 2000, and the sessions were in Miami, London, and Singapore. One of my students in London wanted to see a domain controller promotion in action, so I connected my demo laptop to a handy nearby test network, and promoted it. When I got to Singapore, I realized that I’d forgotten to demote it, and for a minute there I was feeling pretty foolish!
Why? Because you can’t demote a domain controller unless it’s connected to the domain where you promoted it. Windows Server wants to replicate any local changes off to another DC before it permits your DC to be demoted. If it can’t find another DC to replicate to, it won’t let you run DCPROMO. At least, not to a successful conclusion!
There are other scenarios when a demotion might fail, too. I was recently teaching a Server 2008 class with some revised labs, and we ran into issues demoting a DC that was also a primary DNS server, pointing to itself for DNS. The demotion process would proceed partway and then abort. This can result in a “catch-22” situation where the server is in a sort of purgatory, not a functional DC, but not a functional member server either. Thankfully there’s a solution to these sorts of problems, which we’ll discuss next time. (For those who can’t wait: it involves the DCPROMO parameter “forceremoval.”)
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