A method of work mode negotiation for a core backbone, CB, device, comprising: configuring, by the CB device, a first Ethernet port as a downstream port, wherein a work mode of the first Ethernet port is an Ethernet mode, in which the first Ethernet port receives and forwards Ethernet packets; the method characterised in that it further comprises: transmitting a Mode Negotiate Request packet by the CB device through the first Ethernet port to a port extender, PE, device, wherein the Mode Negotiate Request packet carries a set of identifications of PE devices for cascade; receiving, by the CB device, a Mode Negotiate Reply packet through the first Ethernet port sent by the PE device, and transmitting a Mode Negotiate Acknowledge packet through the first Ethernet port to the PE device; changing, by the CB device, the work mode of the first Ethernet port from the Ethernet mode to a cascade mode, which allows an Ethernet packet received by a port of a PE device of a stacking system to be forwarded to another PE device belonging to the same stacking system, after transmitting the Mode Negotiate Acknowledge packet, wherein the stacking system includes the CB device and the PE device and ports of the PE device are connected to ports of the CB device. configuring, by the CB device, a first Ethernet port as a downstream port, wherein a work mode of the first Ethernet port is an Ethernet mode, in which the first Ethernet port receives and forwards Ethernet packets; the method characterised in that it further comprises: transmitting a Mode Negotiate Request packet by the CB device through the first Ethernet port to a port extender, PE, device, wherein the Mode Negotiate Request packet carries a set of identifications of PE devices for cascade; receiving, by the CB device, a Mode Negotiate Reply packet through the first Ethernet port sent by the PE device, and transmitting a Mode Negotiate Acknowledge packet through the first Ethernet port to the PE device; changing, by the CB device, the work mode of the first Ethernet port from the Ethernet mode to a cascade mode, which allows an Ethernet packet received by a port of a PE device of a stacking system to be forwarded to another PE device belonging to the same stacking system, after transmitting the Mode Negotiate Acknowledge packet, wherein the stacking system includes the CB device and the PE device and ports of the PE device are connected to ports of the CB device. transmitting a Mode Negotiate Request packet by the CB device through the first Ethernet port to a port extender, PE, device, wherein the Mode Negotiate Request packet carries a set of identifications of PE devices for cascade; receiving, by the CB device, a Mode Negotiate Reply packet through the first Ethernet port sent by the PE device, and transmitting a Mode Negotiate Acknowledge packet through the first Ethernet port to the PE device; changing, by the CB device, the work mode of the first Ethernet port from the Ethernet mode to a cascade mode, which allows an Ethernet packet received by a port of a PE device of a stacking system to be forwarded to another PE device belonging to the same stacking system, after transmitting the Mode Negotiate Acknowledge packet, wherein the stacking system includes the CB device and the PE device and ports of the PE device are connected to ports of the CB device.