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Configuration Guide - WLAN-AC

S5700 and S6720 V200R012C00

This document describes the configurations of WLAN, including WLAN Service Configuration, Radio Resource Management, Roaming, WLAN QoS, WLAN Security, WDS, Mesh, Location, Hotspot 2.0, Dual-Link Cold Backup, N+1 Backup.
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Guaranteeing Multicast Data Transmission

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.


  1. Run system-view

    The system view is displayed.

  2. Run igmp-snooping enable

    IGMP snooping is enabled globally.

  3. Run vlan vlan-id

    The VLAN view is displayed.

  4. Run igmp-snooping enable

    IGMP snooping is enabled in the VLAN.

  5. (Optional) Run igmp-snooping version version

    The version of IGMP messages that IGMP snooping can process is specified.

    By default, the device can process IGMPv1 and IGMPv2 messages but cannot process IGMPv3 messages.


    If you set the version of IGMP messages that IGMP snooping can process to IGMPv3, the default Layer 2 multicast data forwarding mode cannot be changed on the device.

  6. (Optional) Run igmp-snooping prompt-leave [ group-policy acl-number ]

    The fast leave function is enabled.

    By default, the fast leave function is disabled.

    If trackside APs are directly connected to the device (each device interface maps one trackside AP), enabling fast leave improves the quality of multicast services during link handovers. If the trackside APs are not directly connected to the device, you cannot configure the fast leave function because this function may interrupt multicast services during link handovers.


    For methods of configuring other Layer 2 multicast parameters, see IGMP Snooping Configuration in the S1720, S2700, S5700, and S6720 V200R012(C00&C20) Configuration Guide - IP Multicast.

Updated: 2018-12-24

Document ID: EDOC1100038361

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