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-rw-r--r-- | Documentation/networking/bridge.rst | 56 |
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diff --git a/Documentation/networking/bridge.rst b/Documentation/networking/bridge.rst index 97936b9564fd..863ad2c8d146 100644 --- a/Documentation/networking/bridge.rst +++ b/Documentation/networking/bridge.rst @@ -177,6 +177,62 @@ on a bridge is disabled by default. After enabling VLAN filtering on a bridge, it will start forwarding frames to appropriate destinations based on their destination MAC address and VLAN tag (both must match). +Multicast +========= + +The Linux bridge driver has multicast support allowing it to process Internet +Group Management Protocol (IGMP) or Multicast Listener Discovery (MLD) +messages, and to efficiently forward multicast data packets. The bridge +driver supports IGMPv2/IGMPv3 and MLDv1/MLDv2. + +Multicast snooping +------------------ + +Multicast snooping is a networking technology that allows network switches +to intelligently manage multicast traffic within a local area network (LAN). + +The switch maintains a multicast group table, which records the association +between multicast group addresses and the ports where hosts have joined these +groups. The group table is dynamically updated based on the IGMP/MLD messages +received. With the multicast group information gathered through snooping, the +switch optimizes the forwarding of multicast traffic. Instead of blindly +broadcasting the multicast traffic to all ports, it sends the multicast +traffic based on the destination MAC address only to ports which have +subscribed the respective destination multicast group. + +When created, the Linux bridge devices have multicast snooping enabled by +default. It maintains a Multicast forwarding database (MDB) which keeps track +of port and group relationships. + +IGMPv3/MLDv2 EHT support +------------------------ + +The Linux bridge supports IGMPv3/MLDv2 EHT (Explicit Host Tracking), which +was added by `474ddb37fa3a ("net: bridge: multicast: add EHT allow/block handling") +<https://lore.kernel.org/netdev/20210120145203.1109140-1-razor@blackwall.org/>`_ + +The explicit host tracking enables the device to keep track of each +individual host that is joined to a particular group or channel. The main +benefit of the explicit host tracking in IGMP is to allow minimal leave +latencies when a host leaves a multicast group or channel. + +The length of time between a host wanting to leave and a device stopping +traffic forwarding is called the IGMP leave latency. A device configured +with IGMPv3 or MLDv2 and explicit tracking can immediately stop forwarding +traffic if the last host to request to receive traffic from the device +indicates that it no longer wants to receive traffic. The leave latency +is thus bound only by the packet transmission latencies in the multiaccess +network and the processing time in the device. + +Other multicast features +------------------------ + +The Linux bridge also supports `per-VLAN multicast snooping +<https://lore.kernel.org/netdev/20210719170637.435541-1-razor@blackwall.org/>`_, +which is disabled by default but can be enabled. And `Multicast Router Discovery +<https://lore.kernel.org/netdev/20190121062628.2710-1-linus.luessing@c0d3.blue/>`_, +which help identify the location of multicast routers. + FAQ === |