The IOS network operating system was created from code written by William Yeager at Stanford University,[3][4][5] which was developed in the 1980s for routers with 256 kB of memory and low CPU processing power.
[7] The IOS command-line interface (CLI) provides a fixed set of multiple-word commands.
Cisco IOS has a monolithic architecture, owing to the limited hardware resources of routers and switches in the 1980s.
[6] The Cisco IOS monolithic kernel does not implement memory protection for the data of different processes.
In the event of an IOS crash, the operating system automatically reboots and reloads the saved configuration.
An Interface Descriptor Block, or simply IDB, is a portion of memory or Cisco IOS internal data structure that contains information such as the IP address, interface state, and packet statistics for networking data.
Physical and logical interfaces on the switch will be referenced with either expanded or abbreviated port description names.
[citation needed] To simplify the process of enlarging the feature-set and reduce the need for network operating system reloads, Cisco introduced universal IOS images, that include all features available for a device and customers may unlock certain features by purchasing an additional software license.
The available feature packs are: IOS images can not be updated with software bug fixes.
To patch a vulnerability in IOS, a binary file with the entire operating system needs to be loaded.
Interim releases – Are usually produced on a weekly basis, and form a roll-up of current development effort.
The Cisco advisory web site may list more than one possible interim to fix an associated issue (the reason for this is unknown to the general public).
[16] At the Black Hat Briefings conference in July 2005, Michael Lynn, working for Internet Security Systems at the time, presented information about a vulnerability in IOS.
[19] With IOS being phased out on devices, IOS-XE adopted many improvements including updated defaults.
In addition, competitive router operating systems that emerged 10–20 years after IOS, such as Juniper's Junos OS, were designed to not have these limitations.
[20] Cisco's response was to develop a completely new operating system that offered modularity, memory protection between processes, lightweight threads, pre-emptive scheduling, ability to independently restart failed processes and massive scale for use in Service Provider networks.
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.