Multicast receiver inform the network that they are interested in receiving packets sent to that group. Hence a multicast distribution tree is constructed at the switch.
IGMP snooping takes place internally on L2 switch and its an optional feature for IGMP. It prevents the hosts from receiving unnecessary multicast traffic (DOS attack).
With IGMP snooping enabled,
-> when a switch hears an IGMP report / Join group from a host the switch adds the host's port number to the multicast list.
-> when the switch hears an IGMP report / Leave group, it removes the host's port from the table entry.
Reference:
IGMPv3 report packet - pcap
http://en.wikipedia.org/wiki/Multicast
http://en.wikipedia.org/wiki/IGMP
http://en.wikipedia.org/wiki/IGMP_snooping
http://www-net.cs.umass.edu/kurose/network/mcast/mcast.htm
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