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In all versions of Cisco IOS, packet routing and forwarding ( switching) are distinct functions. In the event of an IOS crash, the operating system automatically reboots and reloads the saved configuration. The disadvantage of the IOS architecture is that it increases the complexity of the operating system, data corruption is possible as one process can write over the data of another, and one process can destabilize the entire operating system or even cause a software-forced crash. This architecture was implemented by Cisco in order to ensure system performance and minimize the operational overheads of the operating system. IOS does however support aliasing of duplicated virtual memory contents to the same physical memory. Therefore the addressable memory is limited to the physical memory of the network device on which the operating system is installed.
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The Cisco IOS kernel does not perform any memory paging or swapping. The entire physical memory is mapped into one virtual address space. The Cisco IOS monolithic kernel does not implement memory protection for the data of different processes. IOS considers each process a single thread and assigns it a priority value, so that high priority processes are executed on the CPU before queued low priority processes, but high priority processes can not interrupt running low priority processes. Instead the process must make a kernel call before other processes get a chance to run. There is no memory protection between processes and IOS has a run to completion scheduler, which means that the kernel does not pre-empt a running process. This means that all processes have direct hardware access to conserve CPU processing time.
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Most builds of IOS include a Tcl interpreter. Through the CLI, the commands available to each privilege level can be defined. All commands are assigned a privilege level, from 0 to 15, and can only be accessed by users with the necessary privilege. "Global configuration mode" provides commands to change the system's configuration, and "interface configuration mode" provides commands to change the configuration of a specific interface. The set available is determined by the "mode" and the privilege level of the current user. The IOS command-line interface (CLI) provides a fixed set of multiple-word commands. For the Nexus switches Cisco developed NX-OS, which is similar to IOS, except that it is Linux-based. Cisco eventually introduced the native mode for chassis, so that they only run one operating system. In early modular chassis network switches from Cisco, modules with layer 3 routing functionalities were separate devices that ran IOS, while the layer 2 switch modules ran CatOS.
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The Cisco Catalyst series would for some time run the CatOS. The company acquired a number of young companies that focused on network switches, such as the inventor of the first Ethernet switch Kalpana, and as a result Cisco switches did not run the IOS. When IOS was developed, Cisco Systems' main product line were routers. Through modular extensions IOS has been adapted to increasing hardware capabilities and new networking protocols. The IOS network operating system was developed in the 1980s for routers that had only 256 kB memory and low CPU processing power.
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Notable exceptions include ASA security products, which run a Linux-derived operating system, carrier routers which run IOS-XR and Cisco's Nexus switch and FC switch products which run Cisco NX-OS.
Main cisco ios command modes code#
Although the IOS code base includes a cooperative multitasking kernel, most IOS features have been ported to other kernels such as QNX and Linux for use in Cisco products. IOS is a package of routing, switching, internetworking and telecommunications functions integrated into a multitasking operating system. ( February 2015) ( Learn how and when to remove this template message)ġ5.9(3)M / August 15, 2019 2 years ago ( )Ĭisco Internetwork Operating System ( IOS) is a family of network operating systems used on many Cisco Systems routers and current Cisco network switches. Please help to improve this article by introducing more precise citations. This article includes a list of general references, but it remains largely unverified because it lacks sufficient corresponding inline citations.