We are planning to upgrade our domain controllers from 2008. We currently have two sites and the plan is to have a domain controller at each site for failover / disaster purposes. Unfortunately at the second site we are only running VMware 5.5. This will be getting upgraded soon, but not in time for the Domain upgrade. As far as I'm aware 5.5 will only support guest operating systems up to 2012. Can I / is it advisable to upgrade the first DC straight to 2016 and then introduce the 2nd DC at the second site as 2012. Or to upgrade the Domain / DC to 2012 at the second site and then introduce the 2016 DC at the primary site. Will the two live happily together until the VCenter upgrade?