3Com Switch 4800G 24-Port Configuration Manual page 555

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Work Mechanism of
IGMP Snooping
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 1/0/2 and Ethernet
1/0/3 of Switch A and Ethernet 1/0/2 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.
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.
Aging timers for dynamic ports in IGMP Snooping and related messages
and actions
Table 55 Aging timers for dynamic ports in IGMP Snooping and related messages and
actions
Timer Description
Router port For each router port, the
aging timer switch sets a timer
initialized to the aging time
of the route port.
Member port When a port joins a
aging timer multicast group, the switch
sets a timer for the port,
which is initialized to the
member port aging time.
The port aging mechanism of IGMP Snooping works only for dynamic ports; a
static port will never age out.
A switch running IGMP Snooping performs different actions when it receives
different IGMP messages, as follows:
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 its router port list and sets an aging timer for this router port.
IGMP Snooping Overview
Message before
expiry
IGMP general query of
which the source address
is not 0.0.0.0 or PIM hello
IGMP membership report
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Action after
expiry
The switch removes
this port from its
router port list.
The switch removes
this port from the
multicast group
forwarding table.