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4.3.5 IGMP Snooping
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 IGMP is also used to periodically check the multicast group for members that are no longer active.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
Type
The IGMP Type codes are shown below:
Type
Meaning
Membership Query (if Group Address is 0.0.0.0)
0x11
Specific Group Membership Query (if Group Address is Present)
0x11
Membership Report (version 2)
0x16
Leave a Group (version 2)
0x17
Membership Report (version 1)
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.
16
Response Time
Group Address (all zeros if this is a query)
Checksum
31
113
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