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14.2 IGMP Protocol Realization
Figure 14-5. IGMP To the Group Protocol Process
IGMP router sends Membership Query to 224.0.0.1 (All host machines group address) in a
period. The host machine will set a delaying timer (Host1 and Host2) for all groups (query
received and interface members existed), every timer is separately set with a random value,
which is generated the highest clock frequency in the host machine. The host machine groups a
member report (Edition 2, TTL is 1) into this group when the timer comes to the time (Ref Host1
Sending Report in Figure 14-5). The host machine will stop timming of its special group and not
send report if it has received a report from another host machine but its own timer has not come
to the specific time (Ref Host2 in the Figure), which will reduce repeat report.
The host machine will send a report to the group if it needs to enter into some specific group
(Ref Host3 in Figure). IGMP will add the group report into multicast group memberships list after
it receives the port. Repeat report will refresh this timer. Router will suppose that there is no local
member if it does not receive the specific report before the timer come to the time, and it needs
not forward group message in neighboring network.
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