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5.6 IGMP Snooping
Theory
Computers and network devices that want to receive multicast transmissions need to inform nearby routers that they will
become members of a multicast group. The Internet Group Management Protocol (IGMP) is used to communicate this
information. IGMP is also used to periodically check the multicast group for members that are no longer active. In the case
where there is more than one multicast router on a sub network, one router is elected as the ‘queried’. This router then
keeps track of the membership of the multicast groups that have active members. The information received from IGMP is
then used to determine if multicast packets should be forwarded to a given sub network or not. The router can check, using
IGMP, to see if there is at least one member of a multicast group on a given subnet work. If there are no members on a sub
network, packets will not be forwarded to that sub network.
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.
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