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User's Manual of ISW-1022M
5.6.5 IGMP Snooping
5.6.5.1 Theory
The Internet Group Management Protocol (IGMP) lets host and routers share information about multicast groups
memberships. IGMP snooping is a switch feature that monitors the exchange of IGMP messages and copies them to the
CPU for feature processing. The overall purpose of IGMP Snooping is to limit the forwarding of multicast frames to only
ports that are a member of the multicast group.
About the Internet Group Management Protocol (IGMP) Snooping
Computers and network devices that want to receive multicast transmissions need to inform nearby routers that they will
become members of a multicast group. The Internet Group Management Protocol (IGMP) is used to communicate this
information. IGMP is also used to periodically check the multicast group for members that are no longer active. In the case
where there is more than one multicast router on a sub network, one router is elected as the 'queried'. This router then
keeps track of the membership of the multicast groups that have active members. The information received from IGMP is
then used to determine if multicast packets should be forwarded to a given sub network or not. The router can check,
using IGMP, to see if there is at least one member of a multicast group on a given subnet work. If there are no members
on a sub network, packets will not be forwarded to that sub network.
Figure 5-54 Multicast Service
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