If you want to force the Exchange server to use a specific GC instead of letting the server automatically choose a GC, you can perform the following steps:
1.Open Exchange management console
2.Right click on “organization configuration” or “server configuration”
3.Select “modify configuration domain controller”
You can view which server Exchange is using by going to “server configuration”. Right click on your server name and select properties. Under the “system settings” tab you will find the servers being used by Exchange.