Guaranteeing Multicast Data Transmission

Vehicle-mounted multimedia devices on moving trains deliver multimedia information services to passengers in multicast mode. Reliable multicast data transmission ensures smooth delivery of multimedia information services. All vehicle-mounted multimedia devices are added to a multicast group. As the train moves ahead, the active link changes frequently. Only the trackside AP and vehicle-mounted AP are aware of the link change. Other ground devices such as switches connected to trackside APs cannot detect the change and fail to forward multicast data. To resolve the problem, IGMP snooping is enabled on the vehicle-mounted AP and ground devices to generate Layer 2 multicast forwarding entries. After switching the active link to a new trackside AP, the vehicle-mounted AP sends a Report message to the trackside AP. The trackside AP forwards the message to the ground device, which then updates the multicast forwarding table accordingly. To prevent loss of multicast packets during a link handover, the vehicle-mounted AP still receives multicast data from the old trackside AP before the multicast flow is switched to the new trackside AP. After receiving multicast data from the new trackside AP, the vehicle-mounted AP sends a Leave message to the old trackside AP. The old trackside AP forwards the Leave message to the ground device, which then stops sending multicast data to the old trackside AP. This mechanism ensures seamless switching of multicast data.

In vehicle-ground fast link handover scenarios, you need to configure multicast on all ground network devices involved in multicast data forwarding according to actual network requirements. This section assumes that the AC participates in Layer 2 multicast data forwarding and uses multicast configuration on the AC as an example. To configure other network devices, see the corresponding configuration document.