Sub-capacity licensing lets you pay for the virtual cores running an IBM product instead of every physical core on the host. That entitlement is not granted by configuration alone. It is granted by evidence, and IBM will only accept evidence from a tool it recognizes. Run an unapproved tool, or run an approved one incorrectly, and the sub-capacity claim collapses to full-capacity charging for the affected period.
The tools IBM will actually accept.
There is no open field here. For sub-capacity reporting, IBM recognizes a defined set of measurement tools, and in practice three of them carry the weight:
- IBM License Metric Tool. ILMT is the default and the one IBM expects to see. It is free to license and built specifically to produce the quarterly sub-capacity report.
- Flexera One ITAM. Accepted as an approved alternative where the estate is already standardized on Flexera for software asset management.
- HCL BigFix Inventory. Accepted as an approved alternative, often where BigFix is already deployed for endpoint management.
A general purpose CMDB, a spreadsheet, or a homegrown script is not on the list. Where a product is metered in VPC, manual counting is no longer permitted at all since Passport Advantage v11, so the tool requirement is firm rather than optional.
The conditions are the same regardless of tool.
Choosing an approved tool is necessary but not sufficient. Every approved tool inherits the identical sub-capacity conditions, and IBM validates against them:
- Deployed within 90 days of the first eligible sub-capacity deployment.
- Running continuously, with no coverage gaps. A gap voids the claim for the period it touches.
- Quarterly reports generated at least once per quarter.
- Reports retained for two years so any quarter in the lookback can be produced on demand.
Switching from ILMT to Flexera or BigFix does not reset these obligations. The clock and the continuity requirement attach to the deployment, not to the tool brand.
How IBM validates the tool output.
During an audit IBM does not simply accept that a tool was running. It validates the output: that the tool version is current and supported, that agent coverage maps to the full host inventory, that products are categorized against the correct bundle, and that a report exists for every quarter in scope. ILMT in particular is known to miscategorize installs, so a report can be technically present and still misstate the position. A buyer side review checks the same things first, because an uncovered host or a misbundled product is a finding waiting to be priced.
Choosing and defending your tool.
The best tool is usually the one your team can run continuously and produce reports from without lapses, because continuity is what the audit tests. If you already operate Flexera or BigFix at scale, an approved alternative can be cleaner than bolting on ILMT. Whatever you run, the defense is the same: prove deployment timing, prove unbroken coverage, prove quarterly reporting, and reconcile the reported counts to your entitlements before IBM does it for you.