In this paper, we propose it virtual data center (VDC) as unit of resource allocation for multiple tenants in the cloud. VDCs are more desirable than physical data centers because the resources allocated to VDCs can be rapidly adjusted as customers’ needs change. To enable the VDC abstraction, we designed a data center network virtualization architecture called SecondNet. SecondNet is scalable by distributing all the virtual-to-physical mapping, routing, and bandwidth reservation state in server hypervisors. Its it port-switching based source routing (PSSR) further makes SecondNet applicable to arbitrary network topologies using commodity servers and switches. SecondNet introduces a centralized VDC allocation algorithm for virtual to physical mapping with bandwidth guarantee. Simulations demonstrated that our VDC allocation achieves high network utilization and low time complexity. Our implementation and experiments on our testbed demonstrate that we can build SecondNet on top of various network topologies, and SecondNet provides bandwidth guarantee and elasticity, as designed.