We have a multi-domain organisation that has multiple DC's. One of these DC's (in Manchester) does the DHCP for all of the connected sites except one; the Cardiff site that contains a second DC for a second Domain.
We have successfully migrated each of the satellite locations on that second Domain to pick up DHCP from the Manchester DC but pick up DNS from the Cardiff DC. This works fine and has been running without issue for months.
However, due to the ending of an ISP contract, we wanted to stop the Cardiff DC supplying DHCP addresses to machines in the Cardiff site and have them pick up those addresses from Manchester instead on a line set up with our new ISP. So, we set up a valid DHCP scope in Manchester, switched off the DHCP Service in Cardiff.... and nothing happened.
Our ISP can see the DHCP requests being made by the machines in Cardiff and can see them hit the Router in Manchester and be sent to the Manchester DC - but that's it. They never see a response from the Manchester DC to DHCP requests from the Cardiff site!
All other sites request DHCP from Manchester and get an almost instant response, but we just can't get it working from Cardiff. As a temporary measure, we are assigning DHCP from the Cardiff Router instead in order to get rid of the defunct network connection.
Does anyone know of a reason why our DC would simply refuse to assign DHCP addresses to the Cardiff site alone? The DHCP range set up is the same as was set up in Cardiff - and is the same as we are assigning from the Router. All of the settings seem to be right, but it just won't do it and we're not sure why. Clearly the requests are being sent to the Server as our ISP has followed the requests that far, so the problem must be with the DC itself somewhere - but it works everywhere else?????