What is a backup domain controler?

Answer 1

When you use Backup to back up the System State data of a domain controller, you also back up the Active Directory® directory service database. To back up the System State data of a domain controller that is running Active Directory, you can use the same procedure as you would for a server that is not a domain controller. However, you must also back up additional files, such as the Active Directory database and log files, and all other files for the system components and services on which Active Directory depends.

The following Active Directory files are part of a System State data backup of a domain controller. By default, these files are located in the Active Directory folder in %SystemRoot%\Ntds.

Active Directory files to back upFile typeDefinition

Ntds.dit

The Active Directory database.

Edb.chk

The checkpoint file.

Edb*.log

The transaction log files; each file is 10 megabytes (MB).

Res1.log and Res2.log

The reserved transaction log files.

In addition to the System State data, you must also back up the Microsoft Windows® boot partition and system partition when you perform either a Windows backup or a full computer backup of a domain controller.

http://technet.microsoft.com/en-us/library/aa997537(EXCHG.65).aspx