Editing Cisco IOS (section)

== Versioning ==
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 a same way ''a.b'' train are released.{{clarify|date=April 2019}} "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 [[software 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 minimize 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.

=== Trains ===
Cisco says, "A train is a vehicle for delivering Cisco software to a specific set of platforms and features."<ref name="train">{{Citation |title= Cisco IOS and NX-OS Software Reference Guide |work= Cisco White Paper |url= http://www.cisco.com/web/about/security/intelligence/ios-ref.html |access-date= June 18, 2013 |archive-date= June 6, 2013 |archive-url= https://web.archive.org/web/20130606060039/http://www.cisco.com/web/about/security/intelligence/ios-ref.html |url-status= live }}</ref>

==== Until 12.4 ====
Before Cisco IOS release 15, releases were 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 targeted.<ref name="train" />

* 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 [[Software bug|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.<ref name="naming">{{Citation |last= Rullan |first= John |title= Understanding Cisco IOS Naming Conventions |work= Cisco Academy Conference Presentation |year= 2005 |url= https://www.cisco.com/c/dam/en_us/training-events/le21/le34/downloads/689/academy/2005/BRK-101.pdf |access-date= October 10, 2016 |archive-date= February 8, 2016 |archive-url= https://web.archive.org/web/20160208150227/http://www.cisco.com/c/dam/en_us/training-events/le21/le34/downloads/689/academy/2005/BRK-101.pdf |url-status= live }}</ref>{{rp|6}}
* 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.<ref name="naming"/>{{rp|6}}
* The '''S''' – [[Service Provider]] train, runs only on the company's core router products and is heavily customized for Service Provider customers.<ref name="naming"/>{{rp|6}}
* The '''E''' – Enterprise train, is customized for implementation in enterprise environments.<ref name="naming"/>{{rp|6}}
* The '''B''' – [[Broadband Internet access|broadband]] train, supports internet based broadband features.<ref name="naming"/>{{rp|6}}
* The '''X*''' (XA, XB, etc.) – Special Release train, contains one-off releases designed to fix a certain bug or provide a new feature.  These are eventually merged with one of the above trains.<ref name="naming"/>{{rp|15}}

There were 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 provide bug fixes for 18 months.