Could not create NTDS settings yadayadayada on domain controller CN=yadayadayada. The RPC server is unavailable.
This was the error message greeting me (with some meaningful text instead of the yadayadayada of course :)) when I tried to add a backup domain controller to the domain of our test and development network. It was the first time I encountered an error at the DC promotion stage.
I checked the usual things (network connections, privileges, etc) but nothing jumped out at me. Now, if an error dialog mentions RPC, then the usual error is either a DNS error, or a DNS server that has not yet refreshed its zone information.
So I opened the DNS config, and discovered that only 1 NIC of our the new DC was registered in DNS. Our networks are multihomed (3 networks in parallel) and yet only 1 address was registered for the NIC. This was a bit odd. There should be 2 (one network has no DNS on purpose).
A quick ping revealed that one of the NICs had no connectivity. It turned out that the primary DC was a bad network connection, due to some wiggling with the cables. Plugging it all the way and then re-registereing the NICs of the new DC with DNS solved the problem.
I still don’t understand why we got that problem, because the disconnected NIC was last in the binding order, so the new DC should have used the available connection anyway. My guess is that it retrieved the address via DNS, and got the address of the NIC that was disconnected. Ah well. Live and learn.