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YML854 Rev1 NB6, NB6W, NB6Plus4, NB6Plus4W, NB6Plus4WnNB6, NB6W, NB6Plus4, NB6Plus4W, NB6Plus4Wn User Guide
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Local Network – IGMP Snooping
Traditionally, IP packets are transmitted in one of either two ways - Unicast (1 sender to 1 recipient) or
Broadcast (1 sender to everyone on the network). Multicast delivers IP packets to just a group of hosts on the
network.
Without IGMP snooping, multicast traffic is treated in the same manner as broadcast traffic, that is, it is
forwarded to all ports. With IGMP snooping, multicast traffic of a group is only forwarded to ports that have
members of that group. IGMP snooping generates no additional network traffic, allowing you to significantly
reduce multicast traffic passing through your switch.
The figure below shows a simple network connected via this ADSL router. There are four Ethernet clients, one
using USB, and two wireless clients.
Now suppose the video server is the multicast transmitter and host A and D are multicast receivers. If we do
not turn on the IGMP snooping function, the router will forward the multicast traffic to all hosts on all interfaces
and consequently block and interrupt the traffic of USB and wireless users, shown as the following figure.