IOS
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Cisco IOS (originally Internetwork Operating System) is the operating system used on Cisco Systems routers and some network switches (those which don't use CatOS). It is a multi-tasking operating system and provides kernel services such as process scheduling as well as the command line interface and routing software.
In all versions of IOS, routing and forwarding (switching) are distinct functions. Routing and other protocols run as IOS processes and generate a forwarding table, which is used by the forwarding function of the router. On router platforms with software-only forwarding (e.g. Cisco 7200) most traffic handling, including access control list filtering and forwarding, is done at interrupt level using Cisco Express Forwarding or dCEF (Distributed CEF). This means IOS does not have to do a process context switch to forward a packet. In routers with hardware-based forwarding, such as the Cisco 12000 series, IOS computes the forwarding table in software and loads it into the forwarding hardware (such as an ASIC or network processor), which does the actual packet switching function.
IOS has a characteristic command line interface, whose style has been widely copied by other networking software. Unlike most operating systems, which use a command followed by a set of arguments, IOS provides a fixed set of multiple-word commands. The set of available commands is determined by the "mode"; for example, "global configuration mode" provides commands to change the system's configuration, and "interface configuration mode" provides commands to change the configuration of a specific layer 2 interface. A typical command may be "show interface gi0/48" or "no ip cef traffic-statistics". All commands are assigned a privilege level, from 0 to 15, and can only be accessed by users with the necessary privilege.
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IOS versioning numbers
IOS versions are versioned using three numbers and some letters, in the general form x.y(z)aa, where
- x is the major version number of the release
- y is the minor version number
- z is the release number, which begins at zero and increments as new releases in the same x.y train are released
- a is the release train identifier, such as none (which designates the mainline, see below), T (for Technology), E (for Enterprise), S (for Service provider) etc.
For example, release 12.3(0) is the first mainline IOS release of version 12.3. 12.3(1) is the next release, and so on. 12.3(0)T is the first release of the T train, 12.3(1)T the next, and so on.
IOS trains
IOS releases are split into several "trains", each containing a different set of features. Trains more or less map onto distinct markets or groups of customers that Cisco is targeting. The mainline train is designed to be the most stable release the company can offer, and its feature set never expands during its lifetime -- it only ever gets bug fixes. The T (Technology) train gets new features and bug fixes throughout its life, and is therefore less stable than the mainline. The S train (Service provider) runs only on the company's core router products and is heavily customized for Service Provider (carrier / PTT) customers. The E train is customized for Enterprise routers. There are other trains from time to time, designed for specific needs -- for example, the 12.0AA train contained new code required for Cisco's AS5800 product.
IOS Feature sets
Each release also has one or more "feature sets"; for example, IOS releases meant for use on Catalyst switches are available as "standard" versions (providing only basic IP routing), "enhanced" versions, which provide full IPv4 routing support, and "advanced IP services" versions, which provide the enhanced features as well as IPv6 support. See White Paper: Cisco IOS Reference Guide (http://www.cisco.com/warp/public/620/5.shtml)
See also
- MicroPat_iOS
- IOS-XR - Internetwork Operating System
- SAN-OS
- CatOS
- Network Operating System
External links
- Cisco IOS Software Configuration (http://www.cisco.com/univercd/cc/td/doc/product/software/) (Version 10.0 through 12.3)
- Project DOTU to document the undocumented commands in Cisco IOS and CATOS. (http://www.boerland.com/dotu/)de:Internetwork Operating System