Internet Group Management Protocol (Igmp) - Allied Telesis SwitchBlade x3100 Series Manual

5. Internet Group Management Protocol (IGMP)

5.1 Introduction
The Internet Group Management Protocol (IGMP) is used to dynamically register (join or leave) individual hosts
in a multicast group on a particular LAN with a multicast router. Hosts join group memberships by sending
IGMP report messages to the multicast group and leave group memberships by sending leave messages to their
local IP multicast router. The IP multicast router listens to the IGMP messages in the group to determine which
multicast groups are active (meaning there is at least one or more host members). Also, the router periodically
sends out IGMP queries to discover which of the multicast groups are still active on a particular subnet.
The SBx3112, through its learning process, knows which unicast MAC address can be reached via which of its
ports and programs its forwarding database (MAC forwarding table) accordingly, and the forwarding process
uses this to forward received unicast frames. Broadcast and multicast frames are forwarded by the SBx3112 out
of all the ports other than the port the frame was received on. This flooding7 approach is acceptable for a
broadcast frame since that is exactly what broadcast implies. However the same approach used for forwarding
multicast frames (containing the IP multicast packet) is less network bandwidth efficient if only a subset of hosts
have joined the IP multicast group and hence are interested in receiving the multicast traffic.
This deficiency can be overcome and the IP multicasting in a layer 2 switching (LAN switching) networking envi-
ronment can be handled more efficiently by implementing the IGMP snooping capability. In the SBx3112,
IGMP snooping is a feature that allows the product to conserve network bandwidth by limiting the layer 2 for-
warding of IP multicast packets only to the LAN segments that have expressed interest in receiving packets
addressed to a multicast group. This function is performed by monitoring the Internet Group Management Pro-
tocol messages between IP hosts and a multicast router
Software Reference for SwitchBlade x3100 Series Switches (Internet Group Management Pro-
tocol (IGMP))
Introduction
5-1