Issue the Dcdiag on all domain controller servers.
Explanation:
Active directory is a database that holds information of client's authentication for service access in a server. Domain controllers in servers holds these active directories, to replicate client's credentials for quick response to access to the server.
When a server domain can not contact the active directory domain controller, the Dcdiag command is issued on the server to check for connectivity issue to the DNS, AD replication, SYSVOL replication etc.
