A major class of network emulation testbeds is based on the Utah Emulab design: a local cluster of experimental nodes interconnected through Ethernet switches using VLANs. The VLANs are configured dynamically to create multiple concurrent experimental topologies. This cluster architecture allows deterministic testbed operation and therefore repeatable experiments. This paper explores the inter-experiment isolation problem for such testbeds, and in particular how to make the isolation robust against attacks when the testbed is designed to support the most dangerous cyber security experiments.