For example, 12.1(8)E14 is a Rebuild, the 14 indicating the 14th rebuild of 12.1(8)E. Rebuilds are developed to either rapidly remediate a fault, or to mollify Cisco’s customers who do not wish to upgrade to a later major revision as they could already be running crucial infrastructure on their hardware devices, and therefore choose to reduce modifications and the associated risks involved.

Cisco IOS Interim Releases – Interim Releases for Cisco IOS are normally developed on a weekly basis, and form a roll-up of the present developmental efforts. The Cisco advisory web site might have more than one possible interim to remediate a related problem (the purpose for this is not known yet to the general public).

Cisco IOS Maintenance Releases – IOS Maintenance Releases are basically meticulously tested releases which are made available and have improvements and bug fixes. Cisco recommends that it’s customers upgrade to Maintenance releases whereever possible, over Interim and Rebuild releases.

Trains

Cisco defines Trains as, “A train is a vehicle for providing Cisco software to a particular set of platforms and features.”

Cisco IOS Until Version 12.4

Prior to the release of Cisco IOS release 15, releases were divided into numerous trains, each comprising a dissimilar set of features. Trains more or less map onto distinctive markets or customer groups which Cisco is targeting.

  • The mainline train is meant to be the most stable release which Cisco can offer, and therefore it’s feature set is never expanded during its lifetime. Updates are released only to remediate bugs present in the product. The preceding technology train becomes the source for the present mainline train — as an example, the 12.1T train becomes the foundation for the 12.2 mainline. Consequently, to identify the features accessible with a specific mainline release, one can simply look at the previous T train release.
  • The T – Technology train, has access to the newest features and bug fixes during the course of its life, and is hence theoretically less stable as compared to the mainline. (In releases before the Cisco IOS Release 12.0, the P train functioned as the Technology train.) Cisco doesn’t advise usage of T train in any production locations except where there is urgency to execute a specific T train’s new IOS features.
  • The S – Service Provider train, runs only on Cisco’s core routing equipment and is comprehensively tailored for Service Provider clients.
  • The E – Enterprise train, is tailored for deployment in enterprise settings.
  • The B – broadband train, provides support for internet based broadband features.
  • The X* (XA, XB, etc.) – Special Release train, comprises of one-off releases developed to remediate a specific bug or supply a new feature. These are ultimately merged with one of the above trains.

There have been other trains from time to time, developed for particular requirements — for example, the 12.0AA train had new code which was a necessary requirement of Cisco’s AS5800.

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