27-3
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 27-1
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.
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 router port existing in its router port list, the switch resets the aging timer of
this router port.
z
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:
z
Upon receiving an IGMP query, a multicast group member host responds with an IGMP report.
z
When intended to join a multicast group, a host sends an IGMP report to the multicast router to
announce that it is interested in the multicast information addressed to that group.
Upon receiving an IGMP report, the switch forwards it through all the router ports in the VLAN, resolves
the address of the multicast group the host is interested in, and performs the following to the receiving
port:
z
If the port is already in the forwarding table, the switch resets the member port aging timer of the
port.
z
If the port is not in the forwarding table, the switch installs an entry for this port in the forwarding
table and starts the member port aging timer of this port.
