Which protocol handles the database synchronization from Cisco Unified CM to populate the Cisco IM and Presence database?

SIMPLE (SIP for Instant Messaging and Presence Leveraging Extensions) is used for external SIP-based presence federation with third-party systems and is not involved in internal CUCM-to-IM&P database synchronization.

LDAP is the protocol CUCM uses to import users from an external corporate directory into CUCM itself, not the protocol used to push that data onward into the IM and Presence database.

Cisco IM and Presence Server uses XMPP as its core data exchange protocol, and the internal synchronization process that populates the IM and Presence database with user accounts and device information from the CUCM publisher relies on XMPP messaging between the two systems. XMPP provides the real-time, extensible messaging framework that allows the IM and Presence Sync Agent to receive and apply updates from CUCM continuously.

AXL/SOAP is an administrative XML API used by external provisioning tools and applications to read and write CUCM configuration data, and is not the mechanism by which CUCM populates the IM and Presence database.