Editing Cisco IOS (section)

== IOS XR train ==
For Cisco products that required very high availability, such as the [[Cisco CRS-1]], the limitations of a monolithic kernel were not acceptable. In addition, competitive router operating systems that emerged 10–20 years after IOS, such as [[Juniper Networks|Juniper]]'s [[Junos OS]], were designed to not have these limitations.<ref>{{Cite web|url=https://www.juniper.net/us/en/products/network-operating-system/junos-os.html|title=Junos OS &#124; Juniper Networks US|website=Juniper Networks|access-date=2024-04-10|archive-date=2024-04-06|archive-url=https://web.archive.org/web/20240406091952/https://www.juniper.net/us/en/products/network-operating-system/junos-os.html|url-status=live}}</ref> Cisco's response was to develop a completely new operating system that offered modularity, memory protection between processes, lightweight threads, [[Preemption (computing)|pre-emptive scheduling]], ability to independently restart failed processes and massive scale for use in Service Provider networks. The IOS XR development train initially used the [[real-time operating system]] [[microkernel]] ([[QNX]]) and a large part of the IOS [[source code]] was re-written to take advantage of the features offered by the kernel. In 2005 Cisco introduced the Cisco IOS XR network operating system on the [[Cisco 12000|12000 series]] of network routers, extending the microkernel architecture from the CRS-1 routers to Cisco's widely deployed [[core router]]s. As of release 6.x of Cisco IOS XR, QNX was dropped in favor of Linux. Part of the initial work focused on modularity inspired modification of monolithic IOS into modular IOS, which extends the microkernel architecture into the IOS environment, while still providing the software upgrade capabilities. That idea was only tested on Catalyst 6500, got limited exposure and was quickly discontinued as requirements were too high and significantly impaired platform operation.