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Difference Between IOS XE 3 & IOS XE 16

For example I have a WS-C3650-48PS switch stack and when I go to for updated versions of the IOS XE software I can download either IOS XE 3.6.5E or IOS XE 16.1.3.  What is the difference when it comes to a "run of the mill" switch/switch stack?

I have searched quite a bit for the differences but I cannot seem to find anything that truly outlines the differences in the versions.

Thank you in advance!!!



I find it strange that there were up-votes on any of these responses, as no one gave a clear answer to the OP's question.
Basically the winner is "Just upgrade because Cisco said so".

I find is worrying when I see


'most' features will be supported 15 and 16.X when upgrading from 3.X.X


So I dont think this is a simple 'upgrade cause cisco says so'


Also - I dont see a 'mainstream' or recommended release in the 15/16 train

which suggests it not quite the finished article.


Whats the Story with MacSec AES 256 support ? Dont see it anywhere, I was told it would be supported in a couple of months - that was 18 months ago.

can you go from 3.X code to 16.X or later?


Is there any progression in how you have to upgrade? 


Came across this post while searching if I should be installing Denali-16.3.8(MD)* or 3.6.10E(MD) on my Catalyst 3850-24P-S switch. BTW the switch came with cat3k_caa-universalk9.SPA.03.06.06E.152-2.E6

Others have mentioned that new direction of firmware development is 16 and 3 will not be developed. But we still see latest release for 3.6.10E(MD) released on 17-May-2019. This is the latest date across all latest releases


firmware                    |filename                                                                          |release date              |filesize
3.6.10E(MD)               cat3k_caa-universalk9.SPA.03.06.10.E.152-2.E10.bin    17-May-2019            288.12 MB
Denali-16.3.8(MD)*    cat3k_caa-universalk9.16.03.08.SPA.bin                         15-Feb-2019            510.77 MB

* being the Cisco suggested release based on software quality, stability and longevity.
Also there were other versions available as
Everest-16.6.6(MD)   cat3k_caa-universalk9.16.06.06.SPA.bin                           23-Apr-2019           386.10 MB
Fuji-16.9.3(MD)         cat3k_caa-universalk9.16.09.03.SPA.bin                           23-Mar-2019          427.58 MB

The above information is from

I found another blog describing the upgrade process






We had been running 16.3.6 and 16.3.7 on our 3650s without issue. 16.3.8 is another MD so should be good also. We experienced a memory leak on 16.3.5 if I recall.

However, last December, we switched them to 16.6.5 so we could have them run the same code as the incoming 9300s, which won't run 16.3. We were comfortable with this because 16.6.5 is an MD, and it worked well.

We're in the process of upgrading to 16.6.6 and that seems to be working well also, on both models.

My rule of thumb with choosing a code version is "the MD release with the highest number at the end that was released within the last 6 months." By that rule we'd be running 16.3.8 on the 3650s, but again we wanted to keep them consistent with the 9300s which don't have 16.3 available. If there is no MD for a switch (like the 2960L), same rule, just without the MD.