Switch Trunking & IOS Version

[Fezziwig]

Hello, quick question.

I have two Cisco 3850s in production that are trunked together and I am needing to stack a new 3850 to the second. The new switch came in with a newer IOS (16.05.06) but the IOS version of my original switches is quite old (3.05.something). I need to update it to 16.05.06 to match the new switch before I can stack.

Question is, will the IOS disparity between the stacked switch & the trunked, main switch cause any issues? So it'll end up being my main switch with IOS 3.05.something with two stacked switches trunked off of it on IOS 16.06.05. I don't plan on updating the IOS of the main switch at this time as I am trying to cause as little disruption as possible since they are in production 24/7.

Thanks in advance!

[marce1000]

 - As far as the trunking-setup is concerned , it will not cause any issues.

   M.

[Hossam Abdelmagid]

Hello Fezziwig,

As far as I know, different versions should not be a problem for the trunk. The only case that might cause a problem if your old trunk use ISL not dot1q trunk protocol. It's recommended to check the configuration before you upgrade. Most newer Cisco switches and OS doesn't include ISL protocol anymore. For the stack yes it will probably stop the stack. Switch stack should have the same OS version to stack together. You can either update the switches separately to a certain OS version or run the newer OS and upgrade the stack switches together at once. After that you can add new switches with same OS version as you like, but they all should have the same OS version. Hopefully I answered your question, if you have further question please let me know. 

Regards,

Hossam