For a shorter update procedure guide check abbreviated article: Short list of upgrade steps without extensive explanations “Cisco Catalyst 6880-X VSS ISSU Upgrade Steps“
Cisco spoiled us over the years with great and detailed documentation on each technology and hardware component they support. Still, I managed to find a part where documentation is not detailed enough to give you definite number of steps to get things done.
While preparing for software upgrade of Cisco Catalyst 6880-X VSS cluster I stumbled on one of the first examples of outdated and vague procedure for upgrade of Cisco device. Here is my successful ISSU (In-Service Software Upgrade) procedure which I done few days ago. I hope it will help you avoid sweating and hoping that you typed the right thing on a VSS cluster that should not go down at any point 🙂
I included an Acronym Guide at the bottom of the post to guide you trough VSS, ISSU, Cluster, and other mentioned abbreviation which are not described in details here
In my case the environment was Catalyst 6880-X and four 6800ia Fabric Extenders FEX. The same procedure is valid for more on for no FEX extenders.
Get the info on which IOS version is supported to be upgraded with ISSU
Not all IOS images can be upgraded to new IOS versions using In Service procedure to avoid network traffic downtime. In order to get things working, you need to get into Cisco docs and find ISSU supported upgrade matrix document.