Internetwork Operating System

from Wikipedia, the free encyclopedia
Internetwork Operating System Software
developer Cisco Systems, Inc.
License (s) Proprietary
Current  version 15.0 (October 2009)
Architecture (s) PowerPC
IOS homepage

Internetwork Operating System Software ( IOS ) is the operating system for routers and switches from the US company Cisco .


The operating system goes back to Bill Yeager , an employee of Stanford University Medical School , who developed the software around 1980 that enables routers to connect networks of different media and protocols with one another. Until 1984 he worked with Sandra Lerner and Leonard Bosack , the founders of Cisco, to improve this software. When Cisco was founded in 1984, Cisco licensed this software from Yeager. Since then it has been used in different versions and version 15.0 has been available since October 2009.

IOS has also been licensed to other manufacturers in the past. So used z. B. in the 1990s the BRouter of the US company DEC a limited functionality Cisco IOS.


In modern devices, IOS is decompressed from the non-volatile flash memory and loaded into the main memory when the device is switched on , while in older router models such as B. the Cisco 2500 is always executed directly from the flash memory. The latter limits the processing speed because of the comparatively low throughput of flash memory compared to main memory. Once started, it provides the basic routing and / or switching functions. Depending on the so-called feature set , routing protocols such as BGP , OSPF , EIGRP and RIP are supported. Depending on the feature set , other network protocols such as IPv6 , IPX , AppleTalk are also supported. Basic packet filter functions are provided via access control lists .

This is controlled and configured via the command line via Telnet , SSH or a direct serial console connection on the end device. Operation is similar to that of MS-DOS or Unix command lines and includes functions such as auto-completion of an entered command. An enormous help is the question mark, which, if entered anywhere, outputs context-dependent short help. However, web and Windows interfaces are also available. IOS can also be configured to a limited extent via SNMP .

Command prompt:

  • User Mode: Router>
  • Privileged Mode: Router #
  • Global Configuration Mode: Router (config) #
  • Special interface configuration mode: Router (config-if) #

With the user mode only simple queries and various show commands can be executed. The router or switch can be configured in privileged mode and in global configuration mode. Switching to privileged mode can require a password , from there you can switch to global configuration mode. In order to be able to set individual interfaces , you have to switch to the "Interface Configuration Mode" of the respective interface.

Application examples

  • View the current configuration:
show running-config
  • Displays the IOS version, router model, equipment, etc .:
show version
  • View detailed information about all router interfaces:
show interface
  • Brief overview of IP addresses and status of the existing router interface:
show ip interface brief
  • Displaying the current routing table for the respective network protocols:
show ip route
show ipv6 route
show ipx route
show appletalk route
  • Change to privileged mode:
  • Change to the global configuration mode:
configure terminal


Cisco IOS versioning uses numbers and letters. In general, the form from (c) De is used, where D and e are optional identifiers. The form from (ci) De is used for interim releases. The following applies:

  • ab denotes the major release. Current major release version is 15.0
  • c designates the maintenance release number. It starts at 1 and is continuously increased within a major release.
  • D denotes the release train (see below). If this is not specified, it is the mainline train.
  • e is the rebuild number. Creating a rebuild is the regular way to incorporate bug fixes. However, this takes longer than with an interim build, as the bug fixes are subjected to certain quality controls.
  • i is the interim build number. It is only given if a so-called interim build is created for customers or for internal purposes. This procedure is used if a corrected version has to be made available very quickly due to a specific bug. The corrections from the interim releases are usually incorporated into the rebuilds and maintenance releases that follow shortly thereafter. Interim releases are only available upon request from Cisco TAC.

Release designators

Each release receives a release designator who makes a statement about quality and usability.

  • ES - Engineer Special: ES can be understood as a beta version. It contains fixes that were worked out by the developers. These versions are mostly only available from the Cisco TAC. ES releases should only be used if the features contained are not yet available in an LD and GD release.
  • ED - Early Deployment: ED can be understood as a beta version. It is usually used for all releases of the T, X, Y and Z trains. ED releases should only be used if the features contained are not yet available in an LD and GD release.
  • LD - Limited Deployment: LD is used for all releases of the mainline train that have not yet been certified by the GD program.
  • GD - General Deployment: GD releases are certified by the GD program (extensive quality check) and are considered to be usable without restrictions. If possible, GD software should always be used.
  • MD - Maintenance Deployment: Since Cisco abolished the GD program for 12.4 at the end of March 2008, MD replaces the previous GD and LD. This does not apply to 12.3 and earlier major releases.
  • DF - Deferred: If a release is withdrawn by Cisco, e.g. B. because it contains certain bugs , the DF designator is assigned. This software is no longer available for download, is no longer sold and is no longer supported.

Release trains

Cisco IOS releases are divided into several release trains. These differ on the one hand in the features they contain (e.g. S- and E-trains), on the other hand also in terms of their further development (mainline as well as T-, X-, Y- and Z-trains).

  • Mainline: The functional scope of this train, i.e. the features it contains, is not expanded within a major release. From one maintenance release to the next, only bug fixes are integrated. That's why this train is the most stable. If a new major release is created, it is always created directly from the previous T-Train. 12.4 was built from 12.3T, and the next major release will come from 12.4T.
  • T - Technology: This train is used for the actual further development from one major release to the next. Each new maintenance release contains a number of new features, most of which come from the current X, Y and Z trains. Therefore, the T-Train is generally considered to be less stable than the Mainline Train. However, bug fixes are also added to the T-Train.
  • X, Y, Z - Experimental: Most of the new features are first published in such a release and later integrated into the T-Train. A new train is created for each feature (sometimes also for several grouped features), starting with XA, XB, etc. Such releases are typical for new platforms, since support for a new platform is also considered a feature. The first available releases for a new platform therefore always come from such a train.
  • S - Service Provider: This train is adapted to the needs of service providers, e.g. B. ISPs. He integrates z. B. Features like MPLS .
  • E - Enterprise: This train is adapted to the needs of larger companies.
  • B - Broadband: Internet broadband features are supported.

Sometimes trains are also combined. The SE-Train is z. B. a combination of S- and E-Train. Every now and then, additional trains are created as required. For example, 12.0AA includes new code required for Cisco's AS5800.


A feature is a specific iOS characteristic, such as: B. the support of a certain protocol, certain cryptographic functions, telephony functions etc. or a smaller extension of an existing feature. Support for new hardware (router, switch, expansion module, etc.) is also considered a feature within Cisco. The functionality of an IOS operating system can be clearly described by listing the features it contains.

At the moment there are a little over 3750 different features available.

Feature sets

Feature sets are firmly defined compilations of features by Cisco, which are then effectively sold or sold. offered for download. Since IOS is not built in a modular way, it is still possible to obtain a combination of functions that is as suitable as possible for the intended purpose. Starting from a basic feature set - usually referred to as "IP", "IP Only" or "IP Base" - feature sets based on this contain a greater range of functions. However, they usually require more RAM and / or flash memory and This can lead to a higher CPU load. In addition, they are also more expensive.

Starting with version 12.2, "Cisco IOS Packaging" was gradually introduced for all routers and switches. It defines eight different feature sets for routers and seven for Catalyst switches.

See also

Web links

Individual evidence

  2. Types of Cisco IOS Software Releases ( Memento of the original from October 29, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. .  @1@ 2Template: Webachiv / IABot /
  3. Cisco IOS Software General Deployment (GD) Program Retirement .
  4. Cisco IOS Packaging .