When the USN Plug-in is enabled, the server automatically assigns an update sequence number to entries every time a write operation (add, modify, modrdn, or delete) is performed. The USN is stored in the entryUSN operational attribute on the entry; the entryUSN, then, shows the number for the most recent change on any entry.

By default, the entryUSN is unique per back end database instance, so entries in other databases may have the same USN. The nsslapd-entryusn-global parameter changes the assignment of USNs from local to global, that is, from being counted on a single database to being counted for all databases in the topology. The parameter is turned off by default.

A corresponding entry, lastusn, is kept in the root DSE entry, which shows the most recently- assigned USN. In local mode, lastusn shows the most recently- assigned USN per back end database. In global mode, lastusn shows the most recently assigned USN for the entire topology.