Cisco IOS

Internetwork Operating System Software ( IOS) is the operating system of Cisco routers and switches.

The operating system goes back to the staff of the Stanford University Medical School named Bill Yeager, the software developed around 1980, which allows the routers to connect networks of different media and protocols with each other. He worked until 1984, Sandra Lerner and Leonard Bosack, the founders of Cisco, in improving this software together. With the founding of Cisco in 1984, Cisco has licensed software from Yeager. Since then it has been used in different versions and is available in version 15.0 since October 2009.

IOS has been licensed in the past to other manufacturers. For example, the widely used in the 1990s, DEC Brouter a limited functionality in Cisco IOS.

IOS is decompressed in modern devices when the device is switched from the non-volatile flash memory and loaded into the main memory, while it is always executed directly from the flash memory in older router models such as the Cisco 2500. The latter limits the flow rate to main memory because of the relatively low throughput of flash memory. After the start, it provides the basic functions of routing and / or switching available. There are, depending on the so-called feature-set supports, among other routing protocols such as BGP, OSPF, EIGRP and RIP. In addition, depending on the feature set, other network protocols such as IPv6, IPX, AppleTalk supported. About Access Control Lists basic packet filtering functions are provided.

Is controlled and configured this. Using command line via a Telnet, SSH or direct serial console connection on the terminal The operation is similar to that of MS -DOS or Unix command-line and includes features such as auto-completion of commands issued. A huge help is the question mark, which at any point input outputs a context-sensitive Quick Help. But there are also Web and Windows interfaces available. Also via SNMP, you can configure IOS restricted.

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. In privileged mode and in Global Configuration mode, the router or switch can be configured. The change in the privileged mode can require a password, from there you can enter Global Configuration Mode. To set a per-interface must be changed in the " Interface Configuration Mode " of the respective interface.


  • Show the current configuration:

Show running -config View the IOS version, router model and 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:

Enable Change in the global configuration mode:

Configure terminal versioning

The Cisco IOS versioning uses numbers and letters. Generally, the mold is used as ( c) De, wherein d and e are optional identifier. In interim releases from the form (ci) De is used. The following applies:

  • A.b designates the major version ( major release ). Current Major Release Version 15.0
  • C denotes the maintenance release number. It starts at 1 and is increased continuously within a major release.
  • D denotes the release train ( see below). If not specified, it is the Main Line Train.
  • E denotes the Rebuild number. Creating a rebuild is the regular way to incorporate bug fixes. However, this takes longer than an interim build, as bug fixes certain quality checks are carried out.
  • I is the interim build number. It is specified only if for customers or for internal purposes a so-called interim build is created. This procedure is used when very quickly a corrected version must be made ​​available under a specific bugs. The corrections in the interim releases will flow mostly in the short subsequent rebuilds and maintenance releases one. Interim releases are only available on request from Cisco TAC.

Release Designators

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

  • ED - Early Deployment: ED can be understood as a beta version. It is generally used for all releases the T -, X-, Y - and Z - trains. ED Releases should only be used if the features that are included are not yet in a LD and GD release.
  • LD - Limited Deployment: LD is used for all releases of the Main Line Train, which have not been certified by the DG program.
  • GD - General Deployment: GD releases are certified by the DG program ( detailed quality check) and are considered to be fully usable. If possible should always be used DG software.
  • MD - Maintenance Deployment: Since Cisco has the GD program for 12.4 abolished by end of March 2008, MD replaces the previous GD and LD. This does not apply to 12.3 and previous major releases.
  • DF - Deferred: If a release of Cisco withdrawn, for example because it contains some bugs, so the DF- designator is assigned. This software is no longer available for download, is no longer sold and is no longer supported.

Release Trains

Releases of Cisco IOS are split into multiple release trains. These differ on the one hand by the contained features ( eg, S and E trains ), on the other hand concerning their further development ( Mainline and T -, X-, Y - and Z -Trains ).

  • Main Line: The functionality of this Trains, ie the features that are included is not expanded within a major release. From a maintenance release to the next only bug fixes are integrated. Therefore, this Train is the most stable. If a new major release created, the result is always this directly from the previous T -Train. 12.4 was created from 12.3T, and the next major release will arise from 12.4T.
  • T - Technology: This Train is the actual development of a major release to the next. Each new maintenance release contains a number of new features that usually come from the current X, Y and Z Trains. Therefore, the T -Train is generally considered less stable than the Mainline Train. The T -Train but also bug fixes are supplied.
  • X, Y, Z - Experimental: Most new features will be published first in such a release, and later integrated into the T -Train. Pro Feature (partly also for several grouped features), a new train is applied, starting with XA, XB, etc. Such releases are typical for new platforms, as well as the support for a new platform is considered the 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, such as ISPs. It integrates such as features such as MPLS.
  • E - Enterprise Edition: This Train is adapted to the needs of larger companies.
  • B - Broadband: It supports broadband Internet features.

Partial Trains also be combined. The SE -Train is for example a combination of S- and E -Train. Every now and then even more Trains are created as needed. For example 12.0AA includes new code that was needed for Cisco AS5800.


A feature is a specific IOS feature, such as the support of a particular protocol, certain cryptographic functions, telephony functions, etc. or even a smaller extension of the existing features. Also, support for new hardware ( router, switch, extension module, etc. ) is considered Cisco internally as a feature. The functionality of the IOS operating system can be uniquely described by a list of features that are included.

At present, a total of about 3750 various features are available.

Feature sets

Feature sets are predefined Cisco combinations of features, which respectively sold effectively. are offered for download. Because IOS is not modular in itself, is nevertheless so the opportunity to get a suitable for the intended purpose of compiling functions. Starting from a base feature set - usually referred to as " IP", "IP Only" or " IP Base " - contain that build feature sets greater functionality. However, you usually need more RAM and / or flash memory and may lead to a higher CPU load. In addition, they are also more expensive.

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