If you ever encounter an error while trying to create a new domain within a forest saying, “The replication operation encountered a database error,” it makes you sweat a bit. Your brain tells you it will be a nightmare to fix, do I have proper backups to make it happen, and the question “why now” shows up.
One of the critical parts of Active Directory is DFS. It allows you to share same NETLOGON/SYSVOL folders across all Domain Controllers in your Forest. Its health is vital to the functionality of your Active Directory. If it’s broken, a lot of things may not work, and it’s not that easy to tell the status of it. At first sight, everything may seem to work correctly, but if you take a closer look – not so much. It’s great if you find it out by yourself, but not fun if suddenly GPO’s don’t apply to some users, computers, and you find out a year later.