D-Link DES-3326SR User Manual page 101

DES-3326SR Layer 3 Fast Ethernet Switch User's Guide
Internet Group Management Protocol (IGMP)
End users that want to receive multicast packets must be able
to inform nearby routers that they want to become a multicast
group member of the group these packets are being sent to.
The Internet Group Management Protocol (IGMP) is used by
multicast routers to maintain multicast group membership.
IGMP is also used to coordinate between multiple multicast
routers that may be present on a network by electing one of the
multicast routers as the 'querier'. This router then keep track
of the membership of multicast groups that have active
members on the network. IGMP is used to determine whether
the router should forward multicast packets it receives to the
subnetworks it is attached to or not. A multicast router that
has received a multicast packet will check to determine if there
is at least one member of a multicast group that has requested
to receive multicast packets from this source. If there is one
member, the packet is forwarded. If there are no members, the
packet is dropped.
IGMP Versions 1 and 2
Users that want to receive multicast packets need to be able to
join and leave multicast groups. This is accomplished using
IGMP.
Figure 5-15. IGMP Message Format
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