Daren Matthews

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 the same a.b train are released.
• d (omitted from general releases) is the interim build number.
• e (zero, one or two letters) is the 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.

For example, release 12.3(1) is the first mainline Cisco IOS release of version 12.3. 12.3(2) is the next release, and so on. 12.3(1)T is the first release of the T train, 12.3(2)T the next, and so on. Interim builds are candidates for the next release, and are frequently made available by Cisco support as a faster way to provide fixes for bugs before the next release is available. For example, 12.3(1.2)T is the 2nd interim build after release 12.3(1)T.

UPDATE: IOS version 15 introduces a new numbering system.  See this post