Mac Login Slow – Windows Active Directory

The school I support uses Windows Active Directory (AD) for user accounts. We have a mix of Macs and Windows for students and teachers. There is a single Domain Controller (DC) at our building which is connected to district headquarters over a WAN link. This link is often congested. At headquarters there are several other Domain Controllers which back up our DC. Our problem is that Mac logins are often very slow. This is intermittent. Frequently, a student who is unable to login to a given Mac will try on another machine close by and the login will work. I suspect this may have to do with our Macs randomly attempting to obtain login service from one of the off-site domain controllers rather than the local DC. The only relevant documentation I can find from Apple is in this white paper: http://images.apple.com/business/solutions/it/docs/Best_Practices_Active_Directory.pdf ------------------------------------------------------- Site awareness Apple’s AD plug-in is AD site aware. It queries the global catalog for site information and polls the site’s domain controllers. From those that respond, the plug-in chooses two and uses them until a network change occurs or until one of the domain controllers stops responding. ----------------------------------------------------------- Does anyone know precisely how to interpret the above in our context? Is it accurate that the client always picks two domain controllers and uses both? We would like to restrict our Macs to use just our dedicated on-site domain controller and never try to contact the off-site DCs. Does anyone know of a way to do that? Thanks!