The CN context provides information about the forest's replication topology and which domains reside in the current forest. Only one CN context is supposed to be registered per forest; re-registering a server in the CN context causes the old CN context information to be overwritten.
In the event Exchange needs to be reinstalled, Exchange 2000 setup has an installation switch named /disasterrecovery. (In Exchange 5.5, this was the /r switch.) When Setup is run with this switch, Exchange restores its configuration and databases, but does not re-register objects in Active Directory since they are already there. The existing CN context information in Active Directory will continue to point to the same machine. If you don't use the /disasterrecovery switch, the old CN information will be destroyed.
Obviously you'll need backups of the database and will need to remember how the server was originally configured to complete the recovery process, but it is mostly automatic. (Bear in mind that after the recovery process finishes, the databases will not be mounted by default, which is done to keep them from being accidentally overwritten.) Also, when doing the restore, make sure you select Disaster Recovery for the Messaging and Collaboration Services and the Exchange System Management Tools
Remember that if you do this on a completely rebuilt machine, you must need to have the security identifier (SID) on the machine in question be the same as it was previously before attempting to re-add it to the domain in this fashion. If you're simply removing Exchange and reinstalling it, the SID should remain the same.
Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.
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This was first published in September 2004