In-Service Software Upgrade (ISSU) is a process that upgrades an image to another image on a device while the network continues
to forward packets. ISSU helps network administrators avoid a network outage when performing a software upgrade. The images
are upgraded in install mode wherein each package is upgraded individually.
StackWise Virtual (SVL) comprises two switches that are connected together to form one virtual swtich. SVL supports In-Service
Software Upgrades. ISSU on SVL is performed either in a single step or in three-steps.
ISSU supports upgrades, downgrades, and rollbacks.
For more information on configuring Stackwise Virtual, refer Configuring Cisco Stackwise Virtual.
The following steps describe the process that is followed in performing ISSU:
Copy the new image to the standby and active switches.
Unzip the files and copy packages to both the active and standby switches.
Install the packages on the standby switch.
Restart the standby switch.
The standby switch is now upgraded to the new software.
Install the packages on the active switch.
Restart the active switch and switchover the standby to new active switch. After the switchover, the new standby switch will
be up with the new software. The new software image is already installed on the new active switch, hence ISSU is completed.
ISSU Upgrade: 3-Step Work Flow
This workflow involves three steps—add, activate, and commit. After activation, all switches are upgraded to new software
version except that the software is not committed automatically but must be performed manually via the install commit command. The advantage of this approach is the system can be rolled back to a previous software version. The system automatically
rolls back if the rollback timer is not stopped using the install abort-timer-stop or the install commit command. If the rollback timer is stopped, the new software version could be run on the device for any duration and then
rolled back to the previous version.
ISSU Upgrade: 1-Step Work Flow
This workflow involves only one step and helps in optimization. You cannot roll back as the upgrade is committed automatically.