This application provides a load sharing method, a root node device, a leaf node device, and a system. The method includes: A first root node device receives a message sent by the leaf node device, where the message includes a first identifier and configuration information. The first identifier indicates the leaf node device. The configuration information indicates a configuration mode, requested by the leaf node device, of load sharing from a root node device group to the leaf node device. The root node device group includes the first root node device. The first root node device determines, based on the first identifier and the configuration information, a configuration result of the load sharing from the first root node device to the leaf node device. In response to the configuration result being a first result, the first root node device determines that the first root node device is a primary root of the leaf node device. Alternatively, in response to the configuration result being a second result, the first root node device determines that the first root node device is a secondary root of the leaf node device. This method can implement load sharing of multicast traffic.