Basics of Multicasting

• IGMP (Internet Group Membership Protocol) v1 was developed to prevent flooding by requiring hosts to 'opt in' to join a multicast.
• The router periodically queried the host and when it stopped responding the router would stop forwarding multicast traffic to it.
• IGMP v2 added the capability of the host to send a leave message, and also to make a group specific enquiry.
• IGMP Snooping enables a layer 2 switch to examine the layer 3 IGMP messages and selectively forward multicast traffic to individual hosts.

IGMP Query

• In order for IGMP, and thus IGMP Snooping, to function, a multicast router must exist on the network and generate IGMP queries.
• The tables created for Snooping (holding the member ports for each a multicast group) are associated with the querier.
• IGMP general queries are forwarded to all switches involved in IGMP Snooping.