OFELIA is an experimental network designed to offer a diverse OpenFlow-enabled infrastructure to allow Software Defined Networking (SDN) experimentation. OFELIA is currently composed of ten sub-testbeds (called islands), most of them in Europe and one in Brazil. An experimenter get access to a so-called slice; a subset of the testbed resources like nodes and links, including the Openflow programmable switches to carry on an experiment. A new network virtualization tool called VeRTIGO has been recently presented to extend the way isolation is achieved between slices (slicing), allowing each experimenter to instantiate an arbitrary virtual network topology on top of a physical testbed. In this paper we present preliminary results obtained by deploying and using VeRTIGO in an experiment running across several OFELIA islands, which has proven to increase flexibility to experimenters willing to play with novel SDN concepts at large scale.