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Microsoft Breaks Down Windows 10's 'Dual-Scan' Feature

Microsoft shed a little more light this week on Windows 10's so-called "dual-scan" behavior, which has been the source of some consternation among organizations.

Specifically, Microsoft explained the effects of using System Center Configuration Manager (SCCM) with Windows Update for Business deferral policies, and how that combination impacts the dual-scan behavior associated with Windows updates. The dual-scan behavior was actually designed as a feature by Microsoft to let organizations use the Windows Update service and also simultaneously control content updates through the Windows Server Update Services (WSUS) client management tool.

However, some organizations that were trying to control the arrival of Windows updates using WSUS and SCCM got tripped up by an unexpected aspect of the dual-scan capability. It delivered updates that weren't wanted when certain defer settings were used.

In January, Microsoft had described those defer settings as being policies associated with Windows Update for Business, an update service that's geared toward always delivering the latest updates. So when IT pros thought they were deferring Windows updates, they were actually triggering them. Even just checking for the availability of Windows updates would cause them to arrive.

In August, Microsoft announced that it was now possible to use defer policies with WSUS and not trigger unwanted updates to Windows clients. An August update to Windows 10, known as KB4034658, enabled such support for deferral policies with WSUS.

Microsoft hadn't really explained back then how SCCM users would be affected, but this week's announcement contained some details. Microsoft's October cumulative update for Windows 10 version 1703 permits the disabling of dual-scan behavior, but it's a nuanced (and rather contradictory) experience for SCCM (ConfigMgr) users using Windows Update for Business (WUfB) deferral policies. It seems they won't be able to block the dual-scan behavior if using SCCM for configurations. Here's how Microsoft expressed that idea:

If you configure and deploy WUfB deferral policy via ConfigMgr, Dual Scan will be automatically enabled*. That is, "Do not allow update deferral policies to cause scans against Windows Update" will be set to disabled on any ConfigMgr client where the WUfB deferral policy is deployed. Even if you enable "Do not allow update deferral policies to cause scans against Windows Update" at the domain level, that setting will be periodically overwritten by the ConfigMgr client. *Assumes that the "Specify intranet Microsoft update service location" policy is set to Enabled.

Does that statement mean that SCCM users can't use Windows Update for Business without triggering the dual-scan behavior? Apparently, that's not the case. IT pros just need to configure the deferral settings using Group Policy Objects (GPOs), instead of using SCCM, to avoid the dual-scan behavior.

"To use WUfB deferral policies while disabling Dual Scan, use GPO to configure all required settings," Microsoft's announcement explained.

However, if organizations actually want to get the dual-scan behavior, then they can use SCCM to deploy Windows Update for Business policies, Microsoft explained.

About the Author

Kurt Mackie is senior news producer for the 1105 Enterprise Computing Group.