The T train is where new technology gets introduced. When a particular new version gets established (main line code) like when 12.3 got established there is a certain set of hardware platforms and a certain set of features that constitute 12.3 mainline. As new hardware is introduced or as new features are included they appear in the T train (12.3T). The hardware and features put into the T train are included in main line code for the next major new version (features from 12.3T get rolled into 12.4 mainline).
I am not a licensing expert but I believe that you are allowed 12.4 and 12.4T within the same feature set.
This is a complicated question and many people have differing opinions about the answer. Some of my colleagues are very reluctant to run T train code on production routers and I sympathesize and understand their position. On the other hand main line code is not bug free or 100% stable (I have encountered some very interesting bugs in main line code).
I believe that in general the main line code is more stable than the T train code. And if you are going to choose IOS for a router and all the features and functions that you need for it are available in main line code, then you are better off using main line code. But sometimes there is a feature or function that you need and it is only available in T train code. I would not refuse to select an image just because it was T train (I have run T train images on many routers with good success) if the T train had a feature or function that I need. For example several of my customers prefer SSH version 2 to version 1 (version 2 has several security enhancements). In release 12.3 main line code only supported version 1 while the T train supported version 2. So in some cases my customers decided to run the T train so they could get version 2 whle some others said we will wait till version 2 is supported in main line code (which it is in 12.4)
I think that we can establish an approximate hierarchy of code version in terms of desirability:
- code that has been declared GD (General Deployment)
- code that is main line but not GD
- code in the T train (or other trains)
if you can get what you need in a higher level version of code you should do so. But I maintain that T train is sometimes the right choice of code for some routers.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...