Cannot say for sure with the 2960X series, but on the 3750 series the real issue was whether the stack ring software was the same version, which sometimes didn't require the same exact IOS version. That said, even on the 3750 series, you often avoided problems by insuring the IOS was exactly the same across all stack members.
Further, also unsure whether "IOS version has to be the same" means the IOS has to be identical down to the patch level, or whether it just means the same version (i.e. 15.2 vs. 15.2(2) vs. 15.2(2)E#).
Release Format
Cisco IOS Software releases use the format A.B(C)D where:
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A, B, and C are numbers.
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D (if present) is a letter.
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A.B is a major release.
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C is the maintenance version. A higher maintenance number means more bug fixes. Any feature, bug fix, and hardware support available in a particular maintenance version are also available in the next one.
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D, if present, indicates that the release is not a major release, but an extension of a major release. These extensions usually provide new features and new hardware support.
Figure 4. Release Name Components—Cisco IOS Software Release 15M&T Train
The components are:
- Major release number: Indicates the applicable software release family.
- Minor release number: Increases by an increment of 1 for each release that introduces significant changes to the software, support for new hardware platforms, and enhancements and bug fixes for existing features and functions.
- Feature release number: Increases by an increment of 1 for each release that introduces new features or functions. The number resets to 0 for each new minor release.
- Release type identifier: Indicates whether the release is an extended maintenance (M) or standard maintenance (T) release.
- Maintenance rebuild number: Indicates whether the release is a rebuild and, if present, which rebuild it is. This number typically increases by an increment of 1 for each new rebuild. For example, and continuing with the examples in Figure 4, Cisco IOS Software Release 15.0(1)M1 is the first rebuild of Release 15.0(1)M and Cisco IOS Software Release 15.1(1)T2 is the second rebuild of Release 15.1(1)T.
The release-naming conventions for releases from other Cisco IOS Software Release 15 trains—15E, 15S, 15SE, 15SG, and 15SY—are similar to those for the 15M&T train. The release names start with a main release number, followed by major and minor feature release numbers, a release train identifier, and a maintenance rebuild number. Figure 5 shows how these components comprise a release name, using the first maintenance rebuild of Cisco IOS Software Release 15.0(1)SY release as an example.
Figure 5. Release Name Components—Other Cisco IOS Software Release 15 Trains
The components are:
- Main release number: Indicates the applicable software release family.
- Major feature release number: Increases by an increment of 1 for each release that introduces significant changes to the software, support for new hardware platforms, and enhancements and bug fixes for existing features and functions. The number resets to 1 with each new extended maintenance release.
- Minor feature release number: Increases by an increment of 1 for each release that introduces new features or functions. The number resets to 1 for each new major feature release.
- Train identifier: Indicates the applicable software train.
- Maintenance rebuild number: Indicates whether the release is a maintenance rebuild that integrates bug fixes for a limited number of high-severity issues and, if present, which rebuild it is. For example, and continuing with the example in Figure 5, the release is the first rebuild of Release 15.0(1)SY. This number typically increases by an increment of 1 for each new rebuild. Note that some rebuilds may use a different naming convention—a lowercase letter after the minor feature release number or the maintenance rebuild number. For example, Cisco IOS Software Release 15.2(2)E received fixes for a few defects and the resulting rebuild was named 15.2(2a)E. Similarly, Release 15.2(2)E5 received a single fix for a significant defect and the resulting rebuild was named Release 15.2(2)E5a.
BTW, rather than upgrade the stack, another option is to downgrade the new stack unit. (Normally, it's more of an advantage to upgrade, keeping your IOS more current, but if you don't have a maintenance contract allowing for software upgrades, I recall, you always permitted to downgrade.)
