If upgrading from RealPresence Desktop 2.0, the 2.0 MSI package needs to be in the same directory as the new 2.1 MSI package and named "RPD-release_2.0.0_27432.msi" for the isntallation to run properly.
The following is a working installation for updating to RealPresence Desktop 2.1 from RealPresence Desktop 2.0.
msiexec /qn /l* C:\temp\2.1\RPD_install_2.txt /i "C:\temp\2.1\RPDesktop.msi" REINSTALLMODE=vomus REINSTALL=ALL
Release notes: http://downloads.polycom.com/video/realpresence_desktop/rpd_rn_v_2_1.pdf
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