3Com 4007, Implementation Manual, Page: 213 / 684

Key Concepts 213
How Hosts Use IGMP Each host uses IGMP to communicate with the querier in a few different
ways.
Host Membership Reports
Hosts transmit Host Membership Reports ( hereafter called IGMP reports)
in response to queries. A host sends a separate report for each group that
it wants to join or to which it currently belongs. Hosts do not send reports
if they are not or do not want to be group members.
Join Message
Rather than wait for a query, a host can also send an IGMP report on its
own initiative to inform the querier that it wants to begin receiving a
transmission for a specific group. This is called a
join
message. The benefit
is faster transmission linkages.
Leave-Group Messages
Leave-group messages are a type of host message defined in IGMP
version 2. If a host wants to leave an IP multicast group, it issues a
leave-group message. Upon receiving such a message, the querier
determines whether that host is the last group member on the
subnetwork by issuing a
group-specific query .
Leave-group messages lower leave latency — that is, the time between
when the last group member on a given subnetwork or segment sends a
report and when a router or switch stops forwarding traffic for that
group. This process conserves bandwidth. The alternative is for the router
or switch to wait for an aging period to expire before it ceases to forward
the group traffic.
Report Suppression and Effect on Switch Activity
If host A hears an IGMP report from host B for the same group, host A
suppresses (does not transmit) its own report for that group.
This approach was designed to conserve bandwidth, and it works well
with routers because a routing interface only needs to know is that there
is at least one group member in a subnetwork for it to forward group
traffic onto that subnetwork.
However, because a switch that is not operating as the querier must
forward IGMP reports to the querier, a switch must be able to track which
ports lead to the querier, to routers, and to group members, so that it can
forward IGMP reports only to the querier.