ISSU overview
The In-Service Software Upgrade (ISSU) feature upgrades software with a minimum amount of downtime.
ISSU is implemented on the basis of the following design advantages:
Separation of service features from basic functions—Device software is segmented into boot, system, and feature images. The images can be upgraded individually.
Independence between service features—Features run independently. One feature can be added or upgraded without affecting the operation of the system or other features.
Support for hotfix—Patch images are available to fix system bugs without a system reboot.
Hardware redundancy—In an IRF fabric, one member device can be upgraded while other member devices are providing services.
For more information about images, see "Upgrading software."