Specifications

When setting up Mac OS X Server as a PDC, make sure your network doesn’t have
another PDC with the same domain name. The network can have multiple Open
Directory masters, but it can have only one PDC.
Open Directory as a Backup Domain Controller (BDC)
Setting a Mac OS X server as a backup domain controller (BDC) provides failover and
backup for the PDC. The PDC and BDC share Windows client requests for domain login
and other directory and authentication services. If the Mac OS X Server PDC becomes
unavailable, the Mac OS X Server BDC provides domain login and other directory and
authentication services.
The BDC has a synchronized copy of the PDC’s user, group, computer, and other
directory data. The PDC and BDC also have synchronized copies of authentication data.
Mac OS X Server synchronizes the directory and authentication data.
Before setting up Mac OS X Server as a BDC, you must set up the server as an Open
Directory replica. The BDC uses the read-only LDAP directory, Kerberos KDC, and
Password Server of the Open Directory replica.
Mac OS X Server synchronizes the PDC and BDC by updating the Open Directory
replica with changes made to the Open Directory master.
You use Server Admin after installation to make Mac OS X Server an Open Directory
replica and BDC. You can set up multiple BDCs, each on a separate Open Directory
replica server.
Important: You must not have duplicate PDCs on a network.
30 Chapter 1 Directory Services with Open Directory