In recent years, the rapid development of cloud computing brings significant innovation in the whole IT industry, which makes the Internet change from translating information data to translating service directly. In cloud computing, enterprises need build their own data centers as privacy clouds or the backbone of public cloud to deploy new Internet information services. Traditional network structure and control plane mechanism in current data centers have their own inherent limitations in network capacity and price quality. The current practicalities also cannot support network multi tenanting which is essential for cloud computing. Therefore, this paper proposes a novel data center architecture which just uses low-cost commercial programmable switches and servers to build the data center networks; and also gives a flexible virtual network bandwidth management mechanism based on convex optimization, which supports the coordinated work between programmable switches and 2.5 layer agents resident in servers. Through a dynamic bandwidth allocation algorithm, it can provide more preferably supports to the resource virtualization which is very common in cloud computing applications. Experimental results show that the proposed data center network architecture can reduce the building-cost significantly as well as improve the network throughput remarkably, and that the virtual network management mechanism provides a flexible manner in bandwidth allocation.