Different ADs in Exchange Server

Hi,

I've exchange server in multiple zones. In every zone I have separate ADC. the problem is that when I check get-adserversettings

following severs are always changed to different ADC.

DefaultGlobalCatalog
DefaultConfigurationDomainController
DefaultPreferredDomainControllers

I always set them to the ADC where our Database is mounted and control is available. But after few days it changed to other ADC in our environment. How can I fix it to a particular ADC.

Regards,

Anees