Qtech QSW-3900-24-SFP-AC [134/245] How igmp snooping works
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Qtech QSW-3900-24-SFP-AC [134/245] How igmp snooping works

Qtech QSW-3900-48-SFP-DC [134/245] How igmp snooping works
QTECH Software Configuration Manual
10-133
Timer Description Message before expiry Action after expiry
Router port aging
timer
For each router port, the
switch sets a timer initialized
to the aging time of the route
port.
IGMP general query of
which the source address is
not 0.0.0.0 or PIM hello
The switch removes this
port from its router port
list.
Member port aging
timer
When a port joins a multicast
group, the switch sets a timer
for the port, which is
initialized to the member port
aging time.
IGMP membership report
The switch removes this
port from the multicast
group forwarding table.
& Note :
The port aging mechanism of IGMP Snooping works only for dynamic ports; a static
port will never age out.
10.4.3 How IGMP Snooping Works
A switch running IGMP Snooping performs different actions when it receives different IGMP messages, as
follows :
10.4.3.1 When receiving a general query
The IGMP querier periodically sends IGMP general queries to all hosts and routers (224.0.0.1) on the local
subnet to find out whether active multicast group members exist on the subnet.
Upon receiving an IGMP general query, the switch forwards it through all ports in the VLAN except the
receiving port and performs the following to the receiving port :
l If the receiving port is a router port existing in its router port list, the switch resets the
aging timer of this router port.
l If the receiving port is not a router port existing in its router port list, the switch adds it into
its router port list and sets an aging timer for this router port.
10.4.3.2 When receiving a membership report
A host sends an IGMP report to the multicast router in the following circumstances :
l Upon receiving an IGMP query, a multicast group member host responds with an IGMP
report.
l When intended to join a multicast group, a host sends an IGMP report to the multicast
router to announce that it is interested in the multicast information addressed to that group.
Upon receiving an IGMP report, the switch forwards it through all the router ports in the VLAN, resolves
the address of the reported multicast group, and performs the following :
l If no forwarding table entry exists for the reported group, the switch creates an entry, adds
the port as member port to the outgoing port list, and starts a member port aging timer for
that port.
l If a forwarding table entry exists for the reported group, but the port is not included in the
outgoing port list for that group, the switch adds the port as a member port to the outgoing
port list, and starts a member port aging timer for that port.
l If a forwarding table entry exists for the reported group and the port is included in the
outgoing port list, which means that this port is already a member port, the switch resets
the member port aging timer for that port.
& Note :
A switch does not forward an IGMP report through a non-router port. The reason is as
follows : Due to the IGMP report suppression mechanism, if the switch forwards a report
message through a member port, all the attached hosts listening to the reported multicast

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