User`s guide

Signamax Connectivity System 065-7708, 065-7728, 065-7764
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The format of an IGMP packet is shown below:
Figure 2-65
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)
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: