Motorola Netopia 3342N - Manual (Page 100)

Administrator’s Handbook
100
Link: IGMP (Internet Group Management Protocol)
Multicasting is a method for transmitting large amounts of information to many, but not all, computers
over an internet. One common use is to distribute real time voice, video, and data ser vices to the set of
computers which have joined a distributed conference. Other uses include updating the address books of
mobile computer users in the field, or sending out company newsletters to a distribution list.
Since a router should not be used as a passive for warding device, Motorola Netopia® Gateways use a pro-
tocol for for warding multicasting: Internet Group Management Protocol (IGMP).
Motorola Netopia® Gateways suppor t IGMP Version 1, Version 2, or, beginning with Motorola Netopia®
Firmware Version 7.7, Version 3.
See the “Advanced” option in “LAN” on page 49 for more information.
IGMP “Snooping” is a feature of Ethernet layer 2 switches that “listens in” on the IGMP conversation
between computers and multicast routers. Through this process, it builds a database of where the multi-
cast routers reside by noting IGMP general queries used in the querier selection process and by listening to
other router protocols.
From the host point of view, the snooping function listens at a por t level for an IGMP repor t. The switch
then processes the IGMP repor t and star ts for warding the relevant multicast stream onto the host's por t.
When the switch receives an IGMP leave message, it processes the leave message, and if appropriate
stops the multicast stream to that par ticular por t. Basically, customer IGMP messages although processed
by the switch are also sent to the multicast routers.
In order for IGMP snooping to function with IGMP Version 3, it must always track the full source filter state
of each host on each group, as was previously done with Version 2 only when Fast Leave suppor t was
enabled.
IGMP Version 3 supports:
IGMP Source Filtering: the ability for group memberships to incorporate source address filtering. This allows
“Source-Specific Multicast” (SSM). By adding source filtering, a Gateway that proxies IGMP can more selec-
tively join the specific multicast group for which there are interested LAN multicast receivers.
These features require no user configuration on the Gateway.
To configure IGMP options available in Motorola Netopia® Gateways, click the IGMP link.