The domain controllers in your environment are either Windows Server 2008 R1 or R2 and you’re trying to mark an object or subtree as authoritative so that they will be replicated to the other domain controllers by following the instructions provided in:
Mark an Object or Objects as Authoritative
However, you notice that whenever you attempt to mark an object, subtree, or restore the database, you receive the following warning:
Active Instance not set. To set an active instance use "Activate Instance ".
Proceeding with the restore database or restore object or restore subtree will throw the error:
Error parsing Input – Invalid Syntax
The instructions provided is actually missing an important step where you should execute the following:
activate instance ntds
The following is what the restore is supposed to look like:
ntdsutil: activate instance ntds
Active instance set to "ntds".
ntdsutil: authoritative restore
authoritative restore: restore subtree "CN=CDP,CN=Public Key Services,CN=Service
The TechNet article is dated July 15, 2010 and it looks like Microsoft never updated it with the missing step another user pointed out at the bottom so if you run into this issue, execute the missing command prior using the restore command.