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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
114 Switch Management and Operating Concepts
2
Table 5-14. IGMP Type Codes
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.