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IOS XR is a train of Cisco Systems' widely deployed Internetworking Operating System (IOS), used on their high-end Network Converging System(NCS), carrier-grade routers such as the CRS series, 12000 series, and ASR9000 series.
According to Cisco's product literature, IOS XR shares very little infrastructure with the other IOS trains, and is instead built upon a "preemptive, memory protected, multitasking, microkernel-based operating system". The microkernel is provided by QNX.
IOS XR aims to provide the following advantages over the earlier IOS trains:
- Improved high availability (largely through support for hardware redundancy and fault containment methods such as protected memory spaces for individual processes and process restartability)
- Better scalability for large hardware configurations (through a distributed software infrastructure and a two-stage forwarding architecture)
- A package based software distribution model (allowing optional features such as multicast routing and MPLS to be installed and removed while the router is in service)
- The ability to install package upgrades and patches (potentially while the router remains in service)
- A web-based GUI for system management (making use of a generic, XML management interface)
Other significant releases include the following.
- 3.2 – first generally available version for the 12000 router series
- 3.9 – first generally available version for the ASR9000 series
- 5.0 (20 September 2013) – first generally available version for the NCS6000 series, which is based upon a Linux kernel
- 6.1.1 - Introduces support for the 64-bit Linux-based IOS XR operating system on ASR9000 series 
- Cisco IOS XR product literature
- QNX press release confirming use of their microkernel in IOS XR
- Cisco press release announcing CRS-1 and IOS XR
- Cisco's release notes, for the various IOS XR versions
- Release notes for XR 5.0