  The CMS experiment will collect data from the proton-proton collisions
delivered by the Large Hadron Collider (LHC) at a centre-of-mass energy up to
14 TeV. The CMS trigger system is designed to cope with unprecedented
luminosities and LHC bunch-crossing rates up to 40 MHz. The unique CMS trigger
architecture only employs two trigger levels. The Level-1 trigger is
implemented using custom electronics. The High Level Trigger is implemented on
a large cluster of commercial processors, the Filter Farm. Trigger menus have
been developed for detector calibration and for fulfilment of the CMS physics
program, at start-up of LHC operations, as well as for operations with higher
luminosities. A complete multipurpose trigger menu developed for an early
instantaneous luminosity of 10^{32}cm{-2}s{-1} has been tested in the HLT
system under realistic online running conditions. The required computing power
needed to process with no dead time a maximum HLT input rate of 50 kHz, as
expected at startup, has been measured, using the most recent commercially
available processors. The Filter Farm has been equipped with 720 such
processors, providing a computing power at least a factor two larger than
expected to be needed at startup. Results for the commissioning of the
full-scale trigger and data acquisition system with cosmic muon runs are
reported. The trigger performance during operations with LHC circulating proton
beams, delivered in September 2008, is outlined and first results are shown.
