User Guide
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conversation between hosts and routers.
Principle of IGMP snooping
By listening to the conversations between hosts and routers, the switch maintains a map of links which
need IP multicast streams. Multicast streams may be filtered from the links which do not solicit them. An
IGMP-Snooping-disabled layer-2 device will flood multicast traffic to all the ports in a broadcast domain
(or the VLAN equivalent). With IGMP snooping enabled, known multicast traffic will be forwarded to hosts
that have explicitly joined the group. It provides switches with a mechanism to prune multicast traffic from
links that do not contain a multicast listener (an IGMP client). Multicast packet transmission with IGMP
Snooping enabled/disabled:
How IGMP Snooping Works
A switch that runs IGMP snooping performs different actions when receiving different IGMP messages.
When receiving a general query
The IGMP querier periodically sends IGMP general queries to all hosts and routers on the local subnet to
determine which active multicast group members exist on the subnet. After receiving an IGMP general
query, the switch forwards it through all ports in the VLAN (except the port that receives the query) and
performs corresponding actions on the receiving port (resets/enables the age timer).
When receiving a membership report
A host sends an IGMP membership report to the multicast router in the following circumstances:
After receiving an IGMP query, a multicast group member host responds with an IGMP membership
report.
When intended to join a multicast group, a host sends an IGMP membership report to the multicast router
to announce that it wants to join the multicast group. After receiving an IGMP membership report, the
switch forwards it through all the router ports in the VLAN, resolves the address of the reported multicast
group and performs corresponding actions on the receiving port (resets/enables the age timer). A switch
does not forward an IGMP membership report through a non-router port.
When receiving a leave message
When an IGMPv1 host leaves a multicast group, the host does not send an IGMP leave message, so the
switch cannot know immediately that the host has left the multicast group. However, as the aging timer on