Abstract The data acquisition system for the CMS experiment atthe Large Hadron Collider (LHC) will require a large andhigh performance event building network. Several switchtechnologies are currently being evaluated in order to com-pare different architectures for the event builder. One can-didate is Myrinet. This paper describes the demonstratorwhich has been setup to study a small-scale (16x16) eventbuilder based on PCs running Linux connected to Myrinetand Ethernet switches.A detailed study of the Myrinet switch performance hasbeen performed for various traffic conditions, includingthe behaviour of composite switches. Results from eventbuilding studies are presented, including measurements onthroughput, overhead and scaling. Traffic shaping tech-niques have been implemented and the effect on the eventbuilding performance has been investigated. 1 Introduction The Compact Muon Solenoid (CMS) experiment [1] isone of the four experiments at the Large Hadron Collider(LHC), which is currently under construction at CERN andscheduled to start operation in 2005. The experiment willoperate with very high interaction rates in order to accessrare physics events or to collect high statistics of relativelymore frequent phenomena. The detector comprises billionsof channels and will produce on the order of 1 MByte ofdata at each bunch crossing every 25 ns, corresponding toa data stream of 40 TByte/s. These data have to be anal-ysed online for selecting the interesting event candidates.This selection proceeds in two levels of data reduction. The