Qtech QSW-3900-48-Т-AC [179/313] How igmp snooping works
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Qtech QSW-3900-48-Т-AC [179/313] How igmp snooping works

Qtech QSW-3900-48-Т-AC [179/313] How igmp snooping works
QTECH Software Configuration Manual
12-178
Figure 2 IGMP Snooping related ports
Ports involved in IGMP Snooping, as shown in Figure 2, are described as follows :
Router port : A router port is a port on the Ethernet switch that leads switch towards the Layer 3
multicast device (DR or IGMP querier). In the figure, Ethernet 1/0 of Switch A and Ethernet 1/0 of
Switch B are router ports. The switch registers all its local router ports (including static and dynamic
router ports) in its router port list.
Member port : A member port is a port on the Ethernet switch that leads switch towards multicast
group members. In the figure, Ethernet 0/0/1/1 and Ethernet 1/2 of Switch A and Ethernet 0/0/1/1 of
Switch B are member ports. The switch registers all the member ports (including static and dynamic
member ports) on the local device in its IGMP Snooping forwarding table.
Note :
Whenever mentioned in this document, a router port is a port on the switch that
leads the switch to a Layer 3 multicast device, rather than a port on a router.
An IGMP-snooping-enabled switch deems that all its ports on which IGMP general
queries with the source address other than 0.0.0.0 or PIM hello messages are received to be
router ports.
12.4.2.2 Port aging timers in IGMP Snooping and related messages and
actions
Table 1 Port aging timers in IGMP Snooping and related messages and actions
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.
12.4.3 How IGMP Snooping Works
A switch running IGMP Snooping performs different actions when it receives different IGMP messages, as
follows :
12.4.3.1 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 :
If the receiving port is a router port existing in its router port list, the switch resets the
aging timer of this router port.
If the receiving port is not a router port existing in its router port list, the switch adds it into

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