Allied Telesis AT-iMG634 - R2 User's Guide download pdf (Page 276)

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IGMP snooping Old IGMP Snooping Functionality

iMG/RG Software Reference Manual (IGMP)

3-14

Host B sends an unsolicited IGMP Membership report.

The Gateway intercepts the IGMP membership report sent by Host B.

As a multicast entry for this group already exists, the Gateway simply adds the port to the already existing

entry for that multicast group. It also adds another Timeout timer specific for this port to the multicast group.

If another host joins another multicast group or the same multicast group, the same procedures described in

the first and second scenarios are performed, respectively. A new Group entry will be added whenever a new

group has been joined.

Note: In order to maintain group membership, the multicast router sends IGMP queries periodically. This query

is intercepted and forwarded to all ports on the switch. All hosts that are members of the group will

answer that query. The IGMP protocol was designed in such a way that only one member of any group

on any VLAN would have to respond to any given query. But, because the reports are intercepted, the

hosts do not see each other's reports, and thus, all hosts send a report (instead of one per group). These

reports are then forwarded to the router; one report per group from among all received responses.

3.1.3.2.2 Leaving a multicast group

When a host wants to leave group it sends an IGMP Leave message specific for the group it wants to leave.

The IGMP Leave message is captured and if no other devices are known to be joined to that multicast group on

that port - then the multicast stream is removed from that port. If no other ports have hosts joined to the

same multicast group, then the leave messages is forwarded to the multicast router. In this way the multicast

traffic the router is asked to stop sending the multicast stream.

If more than one port has hosts that have joined the multicast group, then the host that sent the IGMP Leave

message is removed from the multicast membership record without forwarding the leave message to the multi-

cast router.

• Time-out interval expiring

When the Time-out Interval expires, the Residential Gateway removes that entry from the multicast member-

ship records and that multicast stream from the associated port - if it is the last entry registered against that

port.

3.1.3.3 Proxy Operation Mode

Proxy mode is the default operational mode for the old IGMP snooping mode. It's possible to force the IGMP

snooping to work in proxy mode via the igmp snooping set mode proxy command.

The Gateway responds to the IGMP Group Specific Query from the Multicast Router based on it's internal

multicast records - replying with an IGMP Membership report for each multicast stream that the hosts that it is

managing are subscribed to.

It also periodically sends IGMP Group Specific Query messages to all ports that are not known multicast router

ports - in order to understand which multicast streams are subscribed to on which ports. The frequency with

which this happens is based upon the Query interval that is configured on the device.

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