IOS, IOS-XE and IOS XR

 IOS

The classical IOS is a monolithic kernel that runs all of the necessary modules in the same memory space.

You will find this IOS on entry level routers like ISR or Enterprise switches like 6500 or 3750.

IOS-XE

IOS XE runs as a system daemon on a “modern Linux platform.”  Cisco also abstracted the system functions out of the main kernel and into separate processes.

IOS XE is a software available for the ASR1K.  it’s like having a Linux with a IOS interface. All the IOS functions are provided by a daemon called IOSD. It is the same interface that the IOS with some minor changes like on the debugs… You could even have access to the Linux interface by typing :

request platform software system shell r0

IOS-XR

IOS XR is what the Mirror Universe version of IOS would look like. Much like IOS XE, IOS XR does away with the monolithic kernel and shared memory space of IOS Classic. XR uses an OS from QNX to serve as the base for the IOS functions.

XR is aimed at the larger service provider platforms like the ASR and CRS series of routers. You can see that in the way that XR can allow multiple routing protocol processes to be executed at the same time in different memory spaces. That’s a big key to the service provider.

What makes IOS XR so different from IOS Classic? That lies in the configuration method. While the CLI may resemble the IOS that you’re used to, Instead of making live config changes on a live system, the running configuration is forked into a separate memory space. Once you have created all the changes that you need to make, you have to perform a sanity check on the config before it can be moved into live production.