Update: IOS Release Numbering

Phillip Remaker provided an excellent explanation of new IOS release numbering rules in a comment on the Did you notice 15.1T is released? post. Here’s a short summary:

  • 15.0(1)M was an exception which consolidated the transition from 12.x rules to 15.x rules.
  • Every new 15.x epoch will start with feature releases (15.1(1)T, 15.1(2)T ...) and end with a mature mainline 15.x(y)M release, which will get bug fixes and maintenance rebuilds.
  • 15.x+1(1)T will appear approximately at the same time as 15.x(y)M and the whole cycle will repeat.


  1. Small detail: IOS 15.1(1)XB versus 15.1(1)T

    15.1(1)XB is a fork without 6VPE and some other features, but with LISP support. Hopefully they will consolidate that in the near future by putting LISP in the T train.
  2. Ah, the X-files are always special 8-)
  3. Well that actually begs the next interesting question. Where do the -X and other branch releases fit into this new naming scheme? Would there ever be a say, 15.1(3)XD and would it be a branch off the 15.1(3)T or a fork off the 15.1(3) mainline?
  4. I can try to guess this one: there's no 15.1(3) mainline (see the description in the blog post). There will be 15.1(x)M, after which there will be no more 15.1(x+1)T releases. One would thus expect the X-files to be a branch off a T release.
Add comment