ISR 4451X lost its SSH Keys after downgrading from 16.9.8 to 15.5(3)

[pescla]

Basically title - has anyone ever had this happen? Upgrading the router worked just fine, but after some call issues i downgraded back to the exact same version as before - suddenly i got "Connection refused" - connected via TELNET, hit "show ssh / show ip ssh" and it said "SSH disabled - generate new keys" (paraphrased) - had to enter crypto key generate command to get SSH back up again. How can this be related to an iOS downgrade?

[Roger Kallberg]

Yes that’s a pretty common thing to happen when you downgrade between a big gap in versions. Has happened to me more times than I care to remember. You’ll have to console in to the router and generate the SSH key for SSH to work again.

[pescla]

Interesting - what do you do if you do not have console connection available? in this case i killed a customers router on the other side of the world (no big deal luckily) - i guess ill have to enable telnet in the future before downgrading

[Roger Kallberg]

That would pose a problem. In general it would be advisable to always keep options at hand if you saw off the branch you’re sitting on. There’s no guarantees that any up- or downgrades will have a positive outcome.