This is handled via the LDAP System Configuration (you can only choose one type of directory - e.g., AD) and first enabling sync. Then you need to change the LDAP Directory and LDAP Authentication configurations for the new AD integration. I believe you can only integrate with a single AD domain; however, you can configure up to 5 separate agreements (DirSync / LDAP Directories) to customize the scope of the search (i.e., configuration of specific OU's within the AD domain). 2 resources to look at are:
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/7x/directry.html
http://blog.ipexpert.com/2010/04/28/cucm-and-active-directory-integration/
Hailey
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