Start with the documentation. Look for Active Directory in the NetBackup Administrator's Guide Volume I. For NetBackup 8.1 the Active Directory information begins on page 878.
It doesn't explicitly answer your question about whether you need an agent on the AD server, but as you read it you'll see that the answer is yes. For example, it tells you how to configure the NetBackup Client service on the AD server.
We need an agent on the system because AD is a database. In fact, it's similar to the Exchange database. Both use the Microsoft JET database engine. In backing up a database, we need to work with VSS to take a snapshot of the db at rest (quiescent).
For some applications (Exchange, SharePoint, SQL Server), you can protect the application with a VMware policy. (Even so, we need an agent on the clients.) This VMWare policy feature does not include an option for Active Directory.
