Network system and data transmission method

A network system, applied to perform data transmission in a data center, the network system comprising:

• a controller, generating flooding information according to a flooding-backbone virtual local area network identifier (f-BVID);
• a first server, comprising a first virtual machine corresponding to a first Tenant ID, wherein the first virtual machine generates and outputs a data packet;
• a second server, comprising a second virtual machine corresponding to the first Tenant ID;
• a first Top-of-Rack (TOR) switch, connected with the first server and receiving the data packet from the first server;
• a second TOR switch, connected with the second server;
• and
• a first spine switch, connected with the first TOR switch and the second TOR switch, configured to receive the data packet from the first TOR switch and transmit the data packet to the second TOR switch according to flooding information,
• wherein when the first TOR switch is unable to transmit the data packet to the second virtual machine due to the first TOR switch occurring lookup fail, the first TOR switch transmits the data packet through a downlink port of the first TOR switch according to a service instance identifier (ISID), and the first TOR switch encapsulates the data packet, sets a customer virtual local area network identifier (CVID) as a first value, and transmits the encapsulated data packet to the first spine switch.