I have migrated from 2003 to 2012, but I have not demoted the 2003 server yet. I tried to join a server to the domain with the DNS of that machine assigned to the IP address of the new 2012 DC. it would not join. when I changed the DNS IP address to the old 2003 server it joined immediately.
Is this due to 2003 not being demoted? I just figured the new DC would have allowed this function to happen without demoting the 2003 server?