Allied Telesis AT-9724TS - Manual (Page 40)

The format of an IGMP packet is shown below:
Figure 6- 15. IGMP Message Format
The IGMP Type codes are shown below:
Type Meaning
0x11 Membership Query (if Group Address is 0.0.0.0)
0x11 Specific Group Membership Query (if Group Address is Present)
0x16 Membership Report (version 2)
0x17 Leave a Group (version 2)
0x12 Membership Report (version 1)
Table 6- 1. IGMP Type Codes
IGMP packets enable multicast routers to keep track of the membership of multicast groups, on their respective subnetworks.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 subnetworks. 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 subnetworks.
IGMP version 2 introduces some enhancements such as a method to elect a multicast querier 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:
Figure 6- 16. IGMP State Transitions
39 Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch
Non-
Member
Delaying
Member
Member
IGMP State
Transitions
Join Group
Start Timer Leave Group
Stop Timer
Timer Expires
Send Response Query Received
Start Timer Another Host
Responds
Stop Timer
Count
Becomes
Zero
Leave
Group
Octets
IGMP Message Format
Group Address (all zeros if this is a query)
0 8 16 31
Type Response Time Checksum