Interlogix IFS NS3502-8P-2S User Manual page 165

IGMP Versions 1 and 2
Multicast groups allow members to join or leave at any time. IGMP provides the method for members
and multicast routers to communicate when joining or leaving a multicast group.
IGMP version 1 is defined in RFC 1112. It has a fixed packet size and no optional data.
The format of an IGMP packet is shown below:
IGMP Message Format
Octets
0 8
Type
The IGMP Type codes are shown below:
Type
0x11
0x11
0x16
0x17
0x12
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.
Figure 4-8-3: IGMP Snooping multicast stream control
16
Response Time
Group Address (all zeros if this is a query)
Meaning
Membership Query (if Group Address is
0.0.0.0)
Specific Group Membership Query (if Group
Address is Present)
Membership Report (version 2)
Leave a Group (version 2)
Membership Report (version 1)
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