1-3
Aging timers for dynamic ports in IGMP Snooping and related messages and actions
Table 1-1
Aging timers for dynamic ports in IGMP Snooping and related messages and actions
Timer
Description
Message before
expiry
Action after expiry
Dynamic router port
aging timer
For each dynamic
router port, the switch
sets a timer initialized
to the dynamic router
port aging time.
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.
Dynamic member port
aging timer
When a port
dynamically joins a
multicast group, the
switch sets a timer for
the port, which is
initialized to the
dynamic member port
aging time.
IGMP membership
report
The switch removes
this port from the IGMP
Snooping forwarding
table.
The port aging mechanism of IGMP Snooping works only for dynamic ports; a static port will never age
out.
How IGMP Snooping Works
A switch running IGMP Snooping performs different actions when it receives different IGMP messages,
as follows:
The description about adding or deleting a port in this section is only for a dynamic port. Static ports can
be added or deleted only through the corresponding configurations. For details, see
Configuring Static
Ports
.
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:
z
If the receiving port is a dynamic router port existing in its router port list, the switch resets the aging
timer of this dynamic router port.
z
If the receiving port is not a dynamic router port existing in its router port list, the switch adds it into
its router port list and sets an aging timer for this dynamic router port.
