Motorola 2247-N8 Specifications download pdf (Page 69)

IP IGMP commands

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

Gateways use a protocol for

forwarding multicasting: Internet Group Management Protocol (IGMP).

Motorola Netopia

Gateways support IGMP Version 1, Version 2, or 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 multicast 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 port level for an IGMP report. The switch then pro-

cesses 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 appropriate 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:

◆ IGMP Snooping – enables the Motorola Netopia

Gateway to “listen in” to IGMP traffic. The Gateway discov-

ers 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.

◆ Robustness – a way of indicating how sensitive to lost packets the network is. IGMP can recover from robust-

ness minus 1 lost IGMP packet. The default value is 2.

◆ 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.

◆ 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 interval is 10 seconds and

must be less than the query interval.

◆ 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 seconds.

◆ Querier Version – select a version of the IGMP Querier: version 1, version 2, or version 3. If you know you will

be communicating with other hosts that are limited to v1 or v2, for backward compatibility, select accordingly;

otherwise, allow the default v3.

☛ NOTE:

IGMP Querier version is relevant only if the Gateway is configured for IGMP forwarding. If any IGMP

v1 routers are present on the subnet, the querier

must use IGMP v1. The use of IGMP v1 must be

administratively configured, since there is no reliable way of dynamically determining whether IGMP

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Motorola 2247-N8 Specifications Page 69

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