A packet transmission method, comprising: receiving, by a first transmission device, a first virtual extensible local area network, VXLAN, packet sent by a first service device through a first VXLAN tunnel, wherein the first VXLAN packet comprises a first VXLAN network identifier, VNI, the first VNI indicates that a broadcast domain, BD, to which the first VXLAN packet sent by the first service device through the first VXLAN tunnel belongs is a first BD, and the first transmission device (TLi) and the first service device are located in a first data center, DC; determining, by the first transmission device, first egress information corresponding to the first BD and a first destination media access control, MAC, address, based on a correspondence between the first BD, the first destination MAC address, and the first egress information, wherein the first destination MAC address indicates a destination address of an Ethernet frame comprised in the first VXLAN packet, and the first egress information comprises a first virtual extensible local area network tunnel end point, VTEP, address and a second VNI; generating, by the first transmission device, a second VXLAN packet based on the first VXLAN packet and the first egress information, wherein the second VXLAN packet comprises the second VNI and the Ethernet frame; and sending, by the first transmission device, the second VXLAN packet to a second transmission device through a second VXLAN tunnel, wherein the second transmission device is located in a second DC, the second DC is different from the first DC, the first VTEP address indicates a port through which the second transmission device connects to the second VXLAN tunnel, and the second VNI indicates that a BD to which the second VXLAN packet sent by the first transmission device through the second VXLAN tunnel belongs is a second BD, wherein the second VNI is different from a third VNI, the third VNI is a VNI comprised in a third VXLAN packet sent by the second transmission device to a second service device through a third VXLAN tunnel, the third VXLAN packet is generated by the second transmission device based on the second VXLAN packet, and the second transmission device and the second service device are located in the second DC. receiving, by a first transmission device, a first virtual extensible local area network, VXLAN, packet sent by a first service device through a first VXLAN tunnel, wherein the first VXLAN packet comprises a first VXLAN network identifier, VNI, the first VNI indicates that a broadcast domain, BD, to which the first VXLAN packet sent by the first service device through the first VXLAN tunnel belongs is a first BD, and the first transmission device (TLi) and the first service device are located in a first data center, DC; determining, by the first transmission device, first egress information corresponding to the first BD and a first destination media access control, MAC, address, based on a correspondence between the first BD, the first destination MAC address, and the first egress information, wherein the first destination MAC address indicates a destination address of an Ethernet frame comprised in the first VXLAN packet, and the first egress information comprises a first virtual extensible local area network tunnel end point, VTEP, address and a second VNI; generating, by the first transmission device, a second VXLAN packet based on the first VXLAN packet and the first egress information, wherein the second VXLAN packet comprises the second VNI and the Ethernet frame; and sending, by the first transmission device, the second VXLAN packet to a second transmission device through a second VXLAN tunnel, wherein the second transmission device is located in a second DC, the second DC is different from the first DC, the first VTEP address indicates a port through which the second transmission device connects to the second VXLAN tunnel, and the second VNI indicates that a BD to which the second VXLAN packet sent by the first transmission device through the second VXLAN tunnel belongs is a second BD, wherein the second VNI is different from a third VNI, the third VNI is a VNI comprised in a third VXLAN packet sent by the second transmission device to a second service device through a third VXLAN tunnel, the third VXLAN packet is generated by the second transmission device based on the second VXLAN packet, and the second transmission device and the second service device are located in the second DC.