Cisco IOS and IOS XE Software differences

[PaulSagar]

Considering we have the latest IOS XE version 17.+ with is supported and not EOL'd, and IOS 15.2E still being supported and an active version gets releases still, and with the software images being moderately similar they have some differences specific to configuration. Would IOS 15.2E be supported alongside IOS XE v17+?
Is there any specific documentation that talks about what are the key differences in terms of configuration?
and as till IOS 15 there were L2 and L3 images separately baked as per appliance, how does it stand for IOS XE?

Regards

[balaji.bandi]

IOS is OLD stable IOS code from Cisco for the OLD Platform.

IOS is repalced with IOS XE in new hardware and platform across cisco devices - where the new models introuduced.

Compatability always there IOS device with IOS XE devices.

IOS have old format and recently they also introudced universal (instead of IP Lite / base / Services so on)

IOS XE complete - universal and License based features works.

[marce1000]

            Ref : https://en.wikipedia.org/wiki/Cisco_IOS_XE#Differences_between_IOS_and_IOS_XE

          >....Would IOS 15.2E be supported alongside IOS XE v17+

 Note that a specific device either uses or supports IOS or IOS-XE train (that  is an exclusive OR)

 M.

[Joseph W. Doherty]

The big difference between (classic) IOS and IOS XE, the former is the OS, the latter is comprised of a small OS, that manages the hardware, and a separate IOS has been changed to run (virtually?) under the small hardware controlling OS.  In theory, the latter IOS should functionally be the same as the like version/release/feature of classic IOS.  In practice, as an IOS XE is running under another hardware controlling OS, differences might be found.  (One big advantage of IOS XE, the controlling OS can run other "things", besides IOS, concurrently, without the need to bind such a "thing" into IOS.

[PaulSagar]

Is the classification of an L2 and L3 image still intact for IOS XE releases? there are CSR1k/C8kv images with a 17.x version but both running IOS XE are either an L3 image or an Edge image. How may I get an L2 image? Possibly a qcow2!

[Joseph W. Doherty]

As another poster might have mentioned, in this thread, newer IOS variants often come in "universal" image which has, within it, all features supported by the IOS, but what you can use depends on licensing restrictions, also controlled by the IOS.  Believe this applied to classis IOS or IOS XE.