The release numbering convention for IOS 15 identifies the specific IOS release, including bug fixes and new software features, similar to previous IOS release families. The figure shows examples of this convention for both the EM release and T release.
Extended Maintenance Release
The EM release is ideal for long-term maintenance, enabling customers to qualify, deploy, and remain on the release for an extended period. The mainline train incorporates features delivered in previous releases plus incremental new feature enhancements and hardware support.
The first maintenance rebuild (for bug fixes only, not new features or new hardware support) of Release 15.0(1)M is numbered 15.0(1)M1. Subsequent maintenance releases are defined by an increment of the maintenance rebuild number (i.e., M2, M3, etc.).
Standard Maintenance Release
The T release is used for short deployment releases ideal for the latest new features and hardware support before the next EM release becomes available. The T release provides regular bug fix maintenance rebuilds, plus critical fix support for network affecting bugs such as Product Security Incident Report Team (PSIRT) issues.
The first planned 15 T new feature release is numbered Release 15.1(1)T. The first maintenance rebuild (for bug fixes only, not new features or new hardware support) of Release 15.1(1)T will be numbered 15.1(1)T1. Subsequent releases are defined by an increment of the maintenance rebuild number (i.e., T2, T3, etc.).