Вышел релиз Mainline (15.0M), начат 15.1T. Release Notes for Cisco IOS Release 15.1T
15.0 считается переходной с 12.x на 15.x, так что 15.1 - это пошло уже полноценное развитие 15.

Касательно жизненного цикла версий (из комментов на ioshints):

Phillip Remaker:

15.0M is a transition between 12 and 15. "M" indicates a long-lived mainline release. It is the first mainline release, long lived and bugfix only.

15.1T starts the ".1 epoch" of releases.

15.1(1)T, (2)T, (3)T will be shorter lived feature releases that each add new features. At some point, the .1 epoch will be feature complete and will graduate to the "M" marker at 15.1(x)M. That M release will be long lived. The value of x is not currently known.

When (or slightly before) 15.1 becomes a mainline, 15.2(1)T will be the first release of the ".2 epoch" of releases. 15.2(1)T, 15.2(2)T will each be shorter-lived feature-add releases that add features. 15.2T will graduate to "M" status once feature complete after some number of releases.

Repeat for each epoch, 15.3, 15.4, etc, etc. The "T" changed to "M" when the train is feature complete.

Logically, M and T are the same software base in this model - the M just serves as a lifecycle marker, like the Ubuntu LTS marker.

The "S" family of releases run a similar process, but all S releases are called "S" regardless of long lived or short lived status. The planned lifecycle of an S release is determined from the release notes.

It is critical to note that the 15.1 T and 15.1 S releases will NOT share the exact branch of code the same way that as in the 12.x families. There will be a lot of parts in common, but you cannot assume that 15.1(x)T and 15.1(x)S will be feature-for-feature or bug-for-bug compatible like you could in the 12 releases.

http://www.cisco.com/web/about/secu...ce/ios-ref.html is a pretty good reference. (Oh, snap, it doesn't cover the IOS 15 shift...)

The docs on the web site are lacking in detail. The "graduate to M" idea is not well covered. The new numbering scheme sought to shy away from "low numbers" on a mainline release - "M" now becomes a marker rather than an independent software release train.