Installation of MSP File in Silent Mode
msiexec /p patchfile.msp REINSTALL=ALL REINSTALLMODE=omus
But if you do not give the command line options of REINSTALL and REINSTALLMODE then it will not patch the existing installation package. What it will do is update the locally cached copy of the MSI database.
If you run the MSP in UI mode then it will patch the installation and will also update the locally cached copy of the installation, because when the dialogs are run they in turn set REINSTALL and REINSTALLMODE.