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4.6.4 IGSP
Overview
IGMP snooping is the process of listening to Internet Group Management Protocol (IGMP) network traffic. IGMP
snooping, as implied by the name, is a feature that allows a network switch to listen in on the IGMP conversation
between hosts and routers.
Principle of IGMP snooping
By listening to the conversations between hosts and routers, the switch maintains a map of which links need which
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 received 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
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