IOS Software Modularity
| Developer | Cisco Systems |
|---|---|
| Working state | Current |
| Source model | Closed source |
| Latest release | 15.6(3)M / July 31, 2016 |
| Available in | English |
| Platforms | The majority of Cisco routers and current Cisco switches |
| Default user interface | Command line interface |
| Official website | Cisco IOS |
Cisco IOS (originally Internetwork Operating System) is a family of software used on most Cisco Systems routers and current Cisco network switches. (Earlier switches ran CatOS.) IOS is a package of routing, switching, internetworking and telecommunications functions integrated into a multitasking operating system. 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 or simulators such as Cisco VIRL.
Not all Cisco products run IOS. Notable exceptions include ASA security products, which run a Linux-derived operating system, and carrier routers which run IOS-XR.
The IOS command line interface provides a fixed set of multiple-word commands. The set available is determined by the "mode" and the privilege level of the current user. "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. All commands are assigned a privilege level, from 0 to 15, and can only be accessed by users with the necessary privilege. Through the CLI, the commands available to each privilege level can be defined.
Most builds of IOS include a Tcl interpreter. Using the Embedded Event Manager feature, the interpreter can be scripted to react to events within the networking environment, such as interface failure or periodic timers.
Available command modes include:
Cisco IOS is versioned using three numbers and some letters, in the general form a.b(c.d)e, where:
Rebuilds – Often a rebuild is compiled to fix a single specific problem or vulnerability for a given IOS version. For example, 12.1(8)E14 is a Rebuild, the 14 denoting the 14th rebuild of 12.1(8)E. Rebuilds are produced to either quickly repair a defect, or to satisfy customers who do not want to upgrade to a later major revision because they may be running critical infrastructure on their devices, and hence prefer to minimise change and risk.
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