Figure 4-8-3: IGMP Snooping multicast stream control
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 16 31
Type Response Time
Checksum
Group Address (all zeros if this is a query)
The IGMP Type codes are shown below:
Type Meaning
0x11
Membership Query (if Group Address is
0.0.0.0)
0x11
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.
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