3Com 4200G 24-Port - Work Mechanism of IGMP Snooping

2-2
Figure 2-2 IGMP Snooping related ports
Router A Switch A
Switch B
Eth1/0/1 Eth1/0/2
Eth1/0/3
Eth1/0/1
Eth1/0/2
Receiver
Receiver
Host A
Host B
Host C
Host D
Source
Multicast packets
Router port
Member port
Ports involved in IGMP Snooping, as shown in
Figure 2-2, are described as follows:
z Router port: A router port is a port on the Layer 3 multicast device (DR or IGMP querier) side of the
Ethernet switch. In the figure, Ethernet 1/0/1 of Switch A and Ethernet 1/0/1 of Switch B are router
ports. A switch registers all its local router ports in its router port list.
z Member port: A member port is a port on the multicast group member side of the Ethernet switch.
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 records all member ports on the local device in the IGMP Snooping
forwarding table.
Port aging timers in IGMP Snooping and related messages and actions
Table 2-1 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 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
Work Mechanism of IGMP Snooping
A switch running IGMP Snooping performs different actions when it receives different IGMP messages,
as follows: