Summary: The Event Builder and Level 3 trigger systems of the CDF experiment at Fermilab are required to process about 300 events per second, with an average event size of $\sim$200 KB. In the event building process the event is assembled from 15 sources supplying event fragments with roughly equal sizes of 12-16 KB. In the subsequent commercial processor-based Level 3 trigger, the events are reconstructed and trigger algorithms are applied. The CPU power required for filtering such a high data throughput rate exceeds 45 000 MIPS. To meet these requirements a distributed and scalable architecture has been chosen. It is based on commodity components: VME-based CPU’s for the data read out, an ATM switch for the event building and Pentium-based personal computers running the Linux operating system for the event processing. Event flow through ATM is controlled by a reflective memory ring. The roughly homogeneous distribution of the expected load allows the use of 100 Mbps Ethernet for event distribution and collection within the Level 3 system. Preliminary results from a test system obtained during the last year are presented.