A controller of a virtual switch system receives an uplink packet forwarded by a SDN switch hosted on a server from a VM (virtual machine) hosted on the server. The controller determines an outgoing interface from at least two uplink interfaces on the SDN switch respectively corresponding to aggregated member ports of a physical switch by using an aggregation algorithm according to the uplink packet. The controller generates a first flow table entry and sends the first flow table entry to the SDN switch, wherein the first flow table entry is to instruct the SDN switch to forward a received uplink packet to the physical switch through the outgoing interface.