Cisco c9800 WLC - In Service Software Upgrade aka ISSU
In Service Software Upgrade (ISSU) is a procedure to accomplish a wireless controller upgrade while packet forwarding continues uninterrupted which increases the network availability and reduces downtime.
ISSU provides a complete image upgrade from one image to another
without network downtime. All AP and client sessions are maintained and the procedure is carried out natively from the controller without the need for an external orchestrator or additional licenses.