1. A network system, comprising: at least one controller and a plurality of forwarding devices, wherein the plurality of forwarding devices forms a plurality of virtual network topologies; wherein the at least one controller is configured to allocate a global label to each of the plurality of virtual network topologies, and send a binding relationship between the global labels and the plurality of virtual network topologies to the plurality of forwarding devices, wherein global labels allocated by the controller to different virtual network topologies of the plurality of virtual network topologies are different; and the plurality of forwarding devices is configured to: receive the binding relationship between the global labels and the virtual network topologies, generate, according to the binding relationship, label forwarding entries comprising a mapping relationship between the global labels and forwarding information bases of the virtual network topologies, and when a first packet received by one forwarding device of the plurality of forwarding devices carries a first global label, the forwarding device finds, by searching according to the first global label, a forwarding information base of a first virtual network topology corresponding to the first global label, and determines that the first packet is to be forwarded by using the forwarding information base of the first virtual network topology, wherein the first virtual network topology is one of the plurality of virtual network topologies.