3Com 4800G Series, Configuration Manual

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IGMP Snooping Overview 555
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.
n
■
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
n
The port aging mechanism of IGMP Snooping works only for dynamic ports; a
static port will never age out.
Work Mechanism of
IGMP Snooping
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.
Table 55 Aging timers for dynamic ports 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.