||This article needs additional citations for verification. (September 2009)|
|Company / developer||Cisco Systems|
|Source model||Closed source|
|Latest stable release||15.2(4)M2 / November 8, 2012|
|Latest unstable release||15.3(1)T / November 29, 2012|
|Supported 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 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.
The IOS Tcl 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.
Cisco IOS is versioned using three numbers and some letters, in the general form a.b(c.d)e, where:
- a is the major version number.
- b is the minor version number.
- c is the release number, which begins at one and increments as new releases in the same a.b train are released. "Train" is Cisco-speak for, "...a vehicle for delivering Cisco software to a specific set of platforms and features.."
- d (omitted from general releases) is the interim build number.
- e (zero, one or two letters) is the release train identifier, such as none (which designates the mainline, see below), T (for Technology), E (for Enterprise), S (for Service provider), XA as a special functionality train, XB as a different special functionality train, etc.
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.
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).
Maintenance releases - Rigorously tested releases that are made available and include enhancements and bug fixes. Cisco recommend upgrading to Maintenance releases where possible, over Interim and Rebuild releases.
Cisco says, "A train is a vehicle for delivering Cisco software to a specific set of platforms and features." Cisco provides a Reference Guide for software release numbering.
Until 12.4 
Before Cisco IOS release 15 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 intended to be the most stable release the company can offer, and its feature set never expands during its lifetime. Updates are released only to address bugs in the product. The previous technology train becomes the source for the current mainline train — for example, the 12.1T train becomes the basis for the 12.2 mainline. Therefore, to determine the features available in a particular mainline release, look at the previous T train release.
- The T - Technology train, gets new features and bug fixes throughout its life, and is therefore potentially less stable than the mainline. (In releases prior to Cisco IOS Release 12.0, the P train served as the Technology train.) Cisco doesn't recommend usage of T train in production environments unless there is urgency to implement a certain T train's new IOS feature.
- The S - Service Provider train, runs only on the company's core router products and is heavily customized for Service Provider customers.
- The E - Enterprise train, is customized for implementation in enterprise environments.
- The B - broadband train, supports internet based broadband features.
- The X* - The XA, XB ... special functionality train, needs to be documented
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.
Since 15.0 
Starting with Cisco IOS release 15 there is just a single train: the M/T train. This train includes both extended maintenance releases and standard maintenance releases. The M releases are extended maintenance releases and Cisco will provide bug fixes for 44 months. The T releases are standard maintenance releases and Cisco will only provides bug fixes for 18 months.
Packaging / feature sets 
Most Cisco products that run IOS also have one or more "feature sets" or "packages", typically eight packages for Cisco routers and five packages for Cisco network switches. For example, Cisco 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.
Each individual package corresponds to one service category, such as
- IP data
- Converged voice and data
- Security and VPN
For additional information about Cisco IOS Packaging see White Paper: Cisco IOS Reference Guide
The exact feature set required for a particular function can be determined using the Cisco Feature Set Browser.
Beginning with the 1900, 2900 and 3900 series of ISR Routers, Cisco has revised the licensing model of IOS. Routers come with IP Base installed, and additional feature pack licenses can be installed as bolt-on additions to expand the feature set of the device. The available feature packs are:
- Data adds features like BFD, IP SLAs, IPX, L2TPv3, Mobile IP, MPLS, SCTP.
- Security adds features like VPN, Firewall, IP SLAs, NAC.
- Unified Comms adds features like CallManager Express, Gatekeeper, H.323, IP SLAs, MGCP, SIP, VoIP.
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. Cisco's IOS software maintains one IDB for each hardware interface in a particular Cisco switch or router and one IDB for each subinterface. The number of IDBs present in a system varies with the Cisco hardware platform type.
In all versions of Cisco IOS, packet routing and forwarding (switching) are distinct functions. Routing and other protocols run as Cisco IOS processes and contribute to the Routing Information Base (RIB). This is processed to generate the final IP forwarding table (FIB, Forwarding Information Base), 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 (CEF) or dCEF (Distributed CEF). This means IOS does not have to do a process context switch to forward a packet. Routing functions such as OSPF or BGP run at the process level. In routers with hardware-based forwarding, such as the Cisco 12000 series, IOS computes the FIB in software and loads it into the forwarding hardware (such as an ASIC or network processor), which performs the actual packet forwarding function.
Cisco IOS has a "monolithic" architecture, which means that it runs as a single image and all processes share the same memory space. There is no memory protection between processes, which means that bugs in IOS code can potentially corrupt data used by other processes. It also has a run to completion scheduler, which means that the kernel does not pre-empt a running process — the process must make a kernel call before other processes get a chance to run. For Cisco products that required very high availability, such as the Cisco CRS-1, these limitations were not acceptable. In addition, competitive router operating systems that emerged 10–20 years after IOS, such as Juniper's JUNOS, were designed not to have these limitations. Cisco's response was to develop a new version of Cisco IOS called IOS XR that offered modularity and memory protection between processes, lightweight threads, pre-emptive scheduling and the ability to independently restart failed processes. IOS XR uses a 3rd party real-time operating system microkernel (QNX), and a large part of the current IOS code was re-written to take advantage of the features offered by the new kernel. But the microkernel architecture removes from the kernel all processes that are not absolutely required to run in the kernel, and executes them as processes similar to the application processes. Through this method, IOS XR is able to achieve the high availability desired for the new router platform. Thus IOS and IOS XR are very different codebases, though related in functionality and design. In 2005, Cisco introduced IOS XR on the Cisco 12000 series platform, extending the microkernel architecture from the CRS-1 to Cisco's widely deployed core router.
In 2006, Cisco has made available IOS Software Modularity which extends the QNX microkernel into a more traditional IOS environment, but still providing the software upgrade capabilities that customers are demanding. It is currently available on the Catalyst 6500 enterprise switch.
Security and vulnerabilities 
Cisco IOS has been proven vulnerable to buffer overflows and other problems that have afflicted other operating systems and applications.
Because the IOS needs to know the cleartext password for certain uses, (e.g., CHAP authentication) passwords entered into the CLI by default are weakly encrypted as 'Type 7' ciphertext, such as "Router(config)#username jdoe password 7 0832585B1910010713181F". This is designed to prevent "shoulder-surfing" attacks when viewing router configurations and is not secure - they are easily decrypted using software called "getpass" available since 1995, or "ios7crypt", a modern variant, although the passwords can be decoded by the router using the "key chain" command and entering the type 7 password as the key, and then issuing a "show key" command; the above example decrypts to "stupidpass". However, the program will not decrypt 'Type 5' passwords or passwords set with the enable secret command, which uses salted MD5 hashes.
Note: Cisco recommends that all Cisco IOS devices implement the authentication, authorization, and accounting (AAA) security model. AAA can use local, RADIUS, and TACACS+ databases. However, a local account is usually still required for emergency situations.