In this paper we present the design and performance evaluation of a network emulation cluster built with commodity PCs and network switches. Each emulated node runs inside its own virtual machine and is complete with a kernel and device drivers and the virtual machine monitor ensures isolation between the emulated nodes, fair access to the resources of the underlying physical node, and high performance. The load of traffic conditioning (emulating packet delays, losses and other characteristics of widearea network links) is shared between all physical nodes by conditioning only the traffic originated by the emulated nodes they host. The above organization results in an emulation testbed that is low cost and scalable while providing strict resource isolation between emulated nodes and high emulation fidelity by allowing the emulation of kernels and other system level software. In this paper we present the main considerations behind the design of our testbed and through detailed performance evaluation we demonstrate that our approach can result in a scalable emulation system with high performance.