Planet GSW-1602SF User Manual page 134

0x16 Membership Report (version 2)
0x17 Leave a Group (version 2)
0x12 Membership Report (version 1)
IGMP packets enable multicast routers to keep track of the membership of multicast groups, on their respective sub
networks. The following outlines what is communicated between a multicast router and a multicast group member using
IGMP.
A host sends an IGMP "report" to join a group
A host will never send a report when it wants to leave a group (for version 1).
A host will send a "leave" report when it wants to leave a group (for 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 sub networks. If there is no response from a particular group, the router assumes that there are no
group members on the network.
The Time-to-Live (TTL) field of query messages is set to 1 so that the queries will not be forwarded to other sub networks.
IGMP version 2 introduces some enhancements such as a method to elect a multicast queried for each LAN, an explicit
leave message, and query messages that are specific to a given group.
The states a computer will go through to join or to leave a multicast group are shown below:
Leave Group
(Stop Timer)
Join Group
(Send Report,
Start Timer)
Delaying Member
IGMP State Transitions
User's Manual of GSW-1602SF / GSW-2404SF
Non-Member
Leave Group
Query Received
(Start Timer)
Idle Member
Report Received
(Stop Timer)
Timer Expried
(Send report)
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