3Com Switch 8800 Advanced Software V5 Configuration Guide

IGMP Snooping Overview 541
and Ethernet 1/0 of Switch B are router ports. A switch registers all its local
router ports in its router port list.
■ Member port: Also known as a listener port, a member port is a port on the
multicast group member side of the Ethernet switch. In the figure, Ethernet
1/1/1 and Ethernet 1/1/2 of Switch A and Ethernet1/1/1 of Switch B are
member ports. The switch records all member ports on the local device in the
IGMP Snooping forwarding table.
n
■ Whenever mentioned in this document, a router port is a port on a switch that
leads the switch to a Layer 3 multicast device, rather than a port on a router.
■ An IGMP-snooping-enabled switch deems 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. For details about PIM hello messages, see
“Configuring PIM Hello Options” on page 589.
Port aging timers in IGMP Snooping and related messages and actions
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
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.
When receiving a membership report
A host sends an IGMP report to the multicast router in the following
circumstances:
Tab le 28 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 an
multicast group, the
switch sets a timer for
the port, which is
initialized to the
member port aging
time
IGMP report The switch removes
this port from the
multicast group
forwarding table