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User’s Manual of FGSW-2620VM / FGSW-2620PVM
97
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
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.