How to understand the following sentence?

[Rps-Cheers]

ISSU is supported only within and between major releases, for example, 17.3.x to 17.3.y, 17.6.x to 17.6.y (within a major release) and 17.3.x to 17.6.x, 17.3.x to 17.9.x (among major releases), that is, for two releases after the current supported release. ISSU is NOT supported within and between minor releases or between minor and major releases, for example 17.4.x to 17.4.y or 17.4.x to 17.5.x or 17.3.x to 17.4.x.

https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/17-2/config-guide/b_wl_17_2_cg/in_service_software_upgrade.html#id_128298

[Leo Laohoo]

My take:  Do not make any attempt to use ISSU or FSU/eFSU -- It only works in "corner cases" or in a lab.  

[Rps-Cheers]

Hahaha, I also feel the same way, but I still can't understand what the Cisco documentation says about the ISSU upgrade method. It's hard to understand, although we basically don't use ISSU.

[balaji.bandi]

ISSU works well, if no config or plain system - not in production. production always tricky as per the many users (myself failed and not really tested in real production my point of view).

[Rps-Cheers]

Although I haven't tried ISSU, it doesn't seem to be suitable for production .

[balaji.bandi]

yes i would advise not to take chance, until you want to help for Cisco for testing by doing volunteering.!

[Leo Laohoo · Accepted solution]

---

@Rps-Cheers wrote:

It's hard to understand

---

There is a "Feedback" option.  Put your comments in.  

All I can say is:  ISSU and FSU/eFSU, is not "worth it".  

I do not want to be the developer's "guinea pig".  If they do not want to test ISSU/FSU/eFSU on their own, then I sure wouldn't either.  

[Rps-Cheers]

great idea, thanks for your reply

[Rich R · Accepted solution]

As to the original question I think it's self explanatory - which part don't you understand?

1. It is only supported in the extended support releases 17.3, 17.6, 17.9

2. You can only use it to go from within one of those releases to another dot release of that version eg. 17.3.1 to 17.3.3 or between those releases eg. 17.3.1 to 17.6.1

3. It's only supported up to 2 releases later eg 17.3.1 to 17.9.1 = supported but 17.3.1 to 17.12.1 not supported because that's 3 releases later but 17.6.1 to 17.12.1 would be supported.

But if you going to use ISSU then read the release notes very carefully and test thoroughly (to make sure it works with your network and configuration) then use at your own risk.  As others have said it can often cause more problems than it solves.

[Rps-Cheers]

@Rich RThanks for you explanation

From what you've said, it's really clear.