Amer Networks SS3GR24i - Manual (Page 52)

SS3GR24i/SS3GR48i Layer3 Stackable Gigabit Switch User’s Guide
Multicast routers use IGMP to manage multicast group
memberships:
• An IGMP “report” is sent by a user’s computer to join a
group
• IGMP version 1 does not have an explicit ‘leave’ message.
Group members have an expiration timer, and if this timer
expires before a query response is returned, the member is
dropped from the group.
• IGMP version 2 introduces an explicit “leave” report.
When a user wants to leave a group, this report is sent to
the multicast router (for IGMP version 2).
• Multicast routers send IGMP queries (to the all-hosts
group address: 224.0.0.1) periodically to see whether any
group members exist on their subnetworks. If there is no
response from a particular group, the router assumes that
there are no group members on the network, and multicast
packets are not forwarded.
The TTL field of query messages is set to 1 so that the queries do
not get forwarded to other subnetworks.
IGMP version 2 introduces a few extensions to IGMP version 1
such as, the election of a single multicast querier for each network,
explicit ‘leave’ reports, and queries that are specific to a particular
multicast group.
The router with the lowest IP address is elected as the querier.
The explicit group leave message is added to decrease latency, and
routers can ask for membership reports from a particular multicast
group ID.
The transition states a host will go through to join or leave a
multicast group are shown in the diagram below.
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