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Administrator's Handbook
IGMP Settings
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 services 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 forwarding device, Motorola Netopia
protocol for forwarding multicasting: Internet Group Management Protocol (IGMP).
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Motorola Netopia
Gateways support IGMP Version 1, Version 2, or, beginning with Motorola Netopia
Firmware Version 7.7, Version 3.
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 mul-
ticast routers reside by noting IGMP general queries used in the querier selection process and by listen-
ing to other router protocols.
From the host point of view, the snooping function listens at a port level for an IGMP report. The switch
then processes the IGMP report and starts forwarding the relevant multicast stream onto the host's
port. When the switch receives an IGMP leave message, it processes the leave message, and if appro-
priate stops the multicast stream to that particular port. 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 support
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 selectively join the specific multicast group for which there are interested LAN multicast receivers.
These features require no user configuration on the Gateway.
You can set the following options:
K IGMP Snooping – enables the Motorola Netopia
way discovers multicast group membership for the purpose of restricting multicast transmissions to
only those ports which have requested them. This helps to reduce overall network traffic from
streaming media and other bandwidth-intensive IP multicast applications.
K Robustness – a way of indicating how sensitive to lost packets the network is. IGMP can recover
from robustness minus 1 lost IGMP packet. The default value is 2.
K Query Interval– the amount of time in seconds between IGMP General Query messages sent by the
querier gateway. The default query interval is 125 seconds.
K Query Response Interval – the maximum amount of time in tenths of a second that the IGMP
Gateway waits to receive a response to a General Query message. The default query response inter-
val is 10 seconds and must be less than the query interval.
K Unsolicited Report Interval – the amount of time in seconds between repetitions of a particular
computer's initial report of membership in a group. The default unsolicited report interval is 10 sec-
onds.
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Gateway to "listen in" to IGMP traffic. The Gate-
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Gateways use a
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