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:
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.
