The CLIC is a lightweight protocol that has recently been proposed for efficient communication in clusters using the Linux Operating System. Besides optimizing communication performance, by reducing the latencies and increasing the bandwidth figures even for short messages, the proposed communication layer also meets other requirements such as multiprogramming, portability, protection against corrupted programs, reliable message delivery, direct access to the network for all applications, etc. In this way, instead of removing the operating system kernel from the critical path and creating a user-level network interface, the operating system support has been optimized to provide reliable and efficient network software, avoiding the TCP/IP protocol stack. The LAM-MPI communication layer has been implemented on top of the proposed protocol and the communication performance has been tested in a cluster of PCs with Linux OS and interconnected with Fast Ethernet.