3Com 7757 Configuration Manual page 451

Figure 110 Working mechanism of IGMPv1
Router A
Ethernet
Host A
(G2)
Query
Report
A host joins in the multicast group in the following procedure:
The IGMP querier (such as DR) periodically multicasts IGMP general group
■
query messages to all the hosts in the shared network segment whose address
is 224.0.0.1.
All hosts in the network receive the query messages. If some hosts (such as
■
Host B and Host C) are interested in the multicast group G1, Host B and Host C
will multicast IGMP host report messages (carrying the address of the multicast
group G1) to declare that they will join in the multicast group G1.
All the hosts and routers in the network receive the IGMP host report messages
■
and get to know the address of the multicast group G1. In this case, if other
hosts in the network want to join in the multicast group G1, they will not send
IGMP host report messages about G1. If some hosts in the network want to
join in another multicast group G2, they will send IGMP host report messages
about G2 to respond to the query messages.
After the query/response process, the IGMP routers get to know that receivers
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corresponding to the multicast group G1 exist in the network, and generate
the (*, G1) multicast forwarding entries, according to which the multicast
information is forwarded.
The data from the multicast source reaches the IGMP router through the
■
multicast routes. If there are receivers in the network connected to the IGMP
router, the data will be forwarded to this network segment and the receiver
hosts receive the data.
IGMP leave packet is not defined in IGMPv1, so when a host leaves a multicast
group, only when a query message times out can the multicast router know that a
host has left the group.
When all the hosts in a network segment have left the multicast group, the branch
corresponding to the related network segment is pruned from the multicast tree.
DR
Router B
Host B Host C
(G1)
Overview
(G1)
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