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 complete multi-tasking operating system and provides kernel services such as process scheduling as well as the command line interface and routing software.
Although IOS itself is capable of routing traffic, and routing protocols (such as BGP) are handled in software, most traffic handling (including access control list filtering and forwarding or switching) is done directly by the switch or router's layer 2 interface controllers using Cisco Express Forwarding or dCEF (Distributed CEF). This allows packets to be forwarded at a much higher rate than would be possible if they were handled by the (typically relatively slow) host CPU. Packets handled by the host CPU are termed "process switched".
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 which 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.
IOS releases are split into several "trains", each containing a different set of features. For example, the IOS 12.3 release is split into the "mainline" release (such as 12.3(10)), intended to be a relatively unchanging stable release, and the "T" train (such as 12.3(11)T), which includes new features not added to the mainline release. 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.
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