In a Cisco Nexus 9000 Series chassis with dual supervisors, you can use the in-service software upgrade (ISSU) feature to upgrade the system software while the system continues to forward traffic. An ISSU uses the existing features of nonstop forwarding (NSF) with stateful switchover (SSO) to perform the software upgrade with no system downtime.
An ISSU is initiated through the command-line interface (CLI) by an administrator. When initiated, an ISSU updates (as needed) the following components on the system:
Supervisor BIOS and nx-os image
Module BIOS and image
In a redundant system with two supervisors, one of the supervisors is active while the other operates in standby mode. During an ISSU, the new software is loaded onto the standby supervisor while the active supervisor continues to operate using the old software. As part of the upgrade, a switchover occurs between the active and standby supervisors, and the standby supervisor becomes active and begins running the new software. After the switchover, the new software is loaded onto the (formerly active) standby supervisor.
For more information on ISSU, including the list of supported platforms, see the Cisco Nexus 9000 Series NX-OS Software Upgrade and Downgrade Guide.