There are two good articles for Cisco VSS Firmware upgrade. Much easier to understand than Cisco documentation. Thanks to the writers.
https://howdoesinternetwork.com/2016/cisco-catalyst-6880-x-vss-issu-upgrade-steps
https://dancwilliams.com/cisco-6807-vss-issu-upgrade/
MY NOTES:
Last time I tried to do an In Service Software Upgrade during the day. Normally, it wouldn’t be a problem, it was a procedure I had done many times before, but this time it made me regret it.
The module that provides the connection between the chassis of the second 6800 chassis occasionally resets itself (it is unfortunate that I do not see it in the log).
When I switchover to the second chassis, the update process got stuck and the uninterrupted process resulted in an interruption.
Once again it is understood that the firmware update should be done at night 🙂
Notes to Myself 