The physical topology of a datacenter network is fundamental as it determines latency, bisection bandwidth, the location and properties of congestion, and the network's ability to tolerate failures. Yet, topology is one of the most difficult factors to control during experimentation: when using a production datacenter or a testbed, the only topology available is the one already deployed. If running on an experimenter's own equipment, rewiring may be possible but is cumbersome and of limited scale. This presents a challenge for controlled experimentation, because such experiments should ideally be run on a range of realistic topologies. To tackle this, we present our work on Topocloud, a system that enables experimenters to construct and modify network topologies in a shared public testbed. Topocloud uses hardware P4 switches to fulfill a dual role: each port can act as either a typical Ethernet MAC-learning switch or as a “virtual wire” that connects ports transparently. We demonstrate that virtual wires in Topocloud can be used to construct low latency network topologies incorporating L2 switches.