Cisco IOS Versions

Before we get into the configuration of a Cisco router in Chapter 7, we should first understand a little more about the Cisco IOS. This is an area that many people find confusing, especially when just starting out. Between the various IOS versions and release codes, it’s easy to understand why.

Note: You are not explicitly required to be familiar with the specifics of IOS versions for either the CCNA or CCDA exams. This information is provided for practical purposes only, and to give you some insight into how IOS images are named.

The Cisco IOS is specialized operating system software that was originally designed for Cisco’s routers, but is now also used on much of their switching equipment. The key thing to understand is that Cisco IOS software is specific to the particular hardware platform that you are using. For example, while a release such as IOS version 12.0 will work across many platforms, the specific images provided for the Cisco 2500 series are not for use on 2600 series routers.

Cisco breaks its IOS software releases into groupings referred to as “feature sets”. Many different feature sets will be available for a given hardware platform, and allow different capabilities depending upon the requirements of a given environment. For example, the basic IP feature set image does not include IPSec encryption capabilities, but the IP Plus IPSec 56 feature set image does. In any network environment, required features will vary according to the protocols, security requirements, and functionality needed. When purchasing or upgrading the IOS for a router, it is important to be sure that you have chosen a feature set image that meets the needs of your environment. Of course, the IOS feature set can always be changed (at a cost) if your requirements change.

Cisco releases three major types of IOS versions – major releases, early deployment (ED) releases, and general deployment (GD) releases.

A major release is developed to ensure high quality and stable software for customer networks. Once issued, no new features are ever added to a major release – changes are limited to bug fixes, which are implemented in the form of maintenance updates. For example, you may come across IOS software version 12.1(3). In this case, the number 12.1 identifies the major release. The bracketed 3 identifies the fact that it is the third maintenance release. When any new bug fixes are applied and integrated into a release, a new maintenance number will be associated with it. In this case, the next would be 12.1(4). Maintenance releases are intended to add to a major release’s stability. They tend to be issued approximately every 8 weeks in the early portion of the major release’s lifecycle.

In order to satisfy customers who require access to new or emerging features and functions, Cisco also releases what are referred to as early deployment (ED) releases. These releases include all of the functionality of the associated major release, but also include additional features (and potentially bug fixes for those features). ED releases always end with a capitalized letter – in most cases, the letter “T”. 12.1(3)T represents major release 12.1, maintenance release 3, and an ED release. In any given maintenance release, new features may also be added. A variety of trailing capitalized letters can refer to an ED release. For example, releases ending in “X” represent a one-time-only release.

The final status that Cisco applies to an IOS version is general deployment. While a major release will be deployed to customers along with updated maintenance versions, a product is not considered to be certified for General Deployment (GD) until it has been qualified through extensive exposure to customer networks, analyzed by Cisco engineering groups, and customer feedback has been assessed. A cross-functional team at Cisco decides when the IOS version receives GD status. In the 12.1 IOS major release, everything from maintenance release 13 forward was granted GD status. Simply, GD status means that the stability of the release is considered proven by Cisco and validated by customer experience.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.