I’ve been working with ASR9k for some time. Even if the ASR9k is a Cisco router, it does not use IOS but IOS XR. IOS XR is a specific software that is available on ASR9k, Cisco 12k and CRS-1.
Currently there is three types of software on Cisco Routers. The classical IOS, the IOS XE and the IOS XR.
Classical IOS is on the market for a long time. I start to work with IOS version 10 on Cisco 2500 routers. You will find this IOS on entry level routers like ISR or Enterprise switches like 6500 or 3750. This IOS is a monolythic OS. That means that all the features are in one file and if one function on the system fail most likely all the system fail. Also that mean that if you want to upgrade the Operating System, you need to reboot the system.
IOS XE is a software available for the ASR1K. In fact, in my point of view, 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
A good book to start with IOS XE is Building Service-Aware Networks written by Muhammad Afaq Khan.
IOS XR is a Carrier Class IOS, the goal is to provide a more stable solution with process mirroring and advanced features. The interface is really different from the classical IOS. For example when you do change on the configuration, you need to validate the changes with a “commit”. It is pretty good because you could multiple changes and the activate all the changes in one command. Also, that allow you to decide when the changes will be activated. And finally, you’ve got the option to roll back the changes. On the configuration side, instead of having the configuration grouped by interface, it is grouped by process… So you’ve got all together, the OSPF config or the PIM config, instead of having a part of the configuration on the interface and a part of the configuration at the process level.
To start with IOS XR, I recommended Cisco IOS XR Fundamentals by Mobeen Tahir, Mark Ghattas, Dawit Birhanu et Syed Natif Nawaz.