I wanted to add a redundant DC to a domain with a secondary DNS, DHCP and use DFS (or DFR-R). The site has a VPN that connects to a central network which provides servers for bespoke services. Therefore, our site has been given a subnet which is reasonable for the long term , however, would get a bit tight if i used DHCP 50/50. Unfortunately requesting a larger subnet would be a difficult task I hear that you can use DHCP Collision Detection for redundancy that is when a DCHP server issues a IP it checks that another machine hasn't already been given it by another DHCP server, however, as you can guess it increases network traffic. I wonder if anyone has used this approach ?