Force Which DC client uses

I have 2000 clients that are authenticating to a DC over a WAN, when there is a DC at the same site as the client. I would like to force these clients to authenticate to a specific 2000 DC. I remember seeing an article about a registry setting that will force the client to use a specific server, but now i cant find it anywhere. Does anybody know how to do this, or where I can find an article about this. Thanks a lot.