You can only used a fixed set of fields for the UserID.
LDAP Attribute for User ID | If synchronization with the LDAP server is enabled, you can choose an LDAP attribute value for the user ID. Choose one of the following values from the drop-down list box: •For Microsoft Active Directory –sAMAccountName –mail –employeeNumber –telephoneNumber –userPrincipalName •For Netscape Sun ONE LDAP Server and OpenLDAP –uid –mail –employeeNumber –telephoneNumber |
http://www.cisco.com/en/US/partner/docs/voice_ip_comm/cucmbe/admin/7_1_2/ccmcfg/b02ldsys.html
Once you have created the sync agreement the only way to change it is to delete the whole agreement and re-create it using the new values you need.
LDAP filtering is to define filters about which/what objects/users to sync into CUCM.
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HTH
java
if this helps, please rate