As network computing become commonplace, the interconnection networks and the communication system software become critical in achieving high performance. Thus, it is essential to systematically assess the features and performance of the new networks. Recently, Myricom has introduced a two-port "E-card" Myrinet/PCI-X interface. In this paper, we present the basic performance of its GM2.1 messaging layer, as well as a set of microbenchmarks designed to assess the quality of MPI implementation on top of GM. These microbenchmarks measure the latency, bandwidth, intra-node performance, computation/communication overlap, parameters of the LogP model, buffer reuse impact, different traffic patterns, and collective communications. We have discovered that the MPI basic performance is close to those offered at the GM. We find that the host overhead is very small in our system. The Myrinet network is shown to be sensitive to the buffer reuse patterns. However, it provides opportunities for overlapping computation with communication. The Myrinet network is able to deliver up to 2000MB/s bandwidth for the permutation patterns.