Upgrade router IOS

[Christian Jorge]

Good morning

We have a cisco router 1921 with IOS 15.2(3).T3.

Searching for a new IOS, we found 15.4(3)M9(ED) and 15.5(3)M7(MD) as candidates

We need no new features, only fix vulnerabilities.

1 - Which IOS do you recommend to consider for this router for upgrading?

2 - The most important part: should I have only substitute 15.2(3)T3 for the new IOS in one-step or do I have to consider intermediary IOSs (ex: 15.3.x)?
I'm afraid of losing config or other surprises.

I could not find any exact procedure for this kind of IOS upgrade

Regards

Christian

[lucaslara]

@Christian Jorge

1 - Which IOS do you recommend to consider for this router for upgrading?

R: All the firmwares are recommend, however i think always is the best use the mostly recent firmware.

2 - The most important part: should I have only substitute 15.2(3)T3 for the new IOS in one-step or do I have to consider intermediary IOSs (ex: 15.3.x)?
I'm afraid of losing config or other surprises.

R: this is just a release, not a version upgrade

[lucaslara]

@Christian Jorge

1 - Which IOS do you recommend to consider for this router for upgrading?

R: All the firmwares are recommend, however i think always is the best use the mostly recent firmware.

2 - The most important part: should I have only substitute 15.2(3)T3 for the new IOS in one-step or do I have to consider intermediary IOSs (ex: 15.3.x)?
I'm afraid of losing config or other surprises.

R: this is just a release, not a version upgrade

[Joseph W. Doherty]

"We need no new features, only fix vulnerabilities."

If you don't need any new features, I would normally recommend keeping to the latest in your IOS train, that stems from your current T release. I'm not 100% positive, but for 15.2T, I think that would be 15.2.4M, and its latest is 15.2.4M11. The bad news, though, is 15.2.4M11 is a couple of years old. I.e. it likely doesn't have all the latest known vulnerabilities patches.

The latest release, that should be the closest to what you have now is 15.4(3)M9(ED). (I suspect, the (ED) should be (MD).

"I'm afraid of losing config or other surprises."

I don't think it likely for such closely related versions. Plus, generally newer IOS versions auto translate even some pretty old syntax to their syntax, if necessary. (Downgrading, though, is much more likely to bump into unknown syntax.) Release notes should note any special upgrade requirements, but on most IOS ISRs, you generally don't bump into special upgrade procedures.