14
Timer Description
Message before expiry
Action after expiry
Dynamic member port
aging timer
When a port dynamically
joins a multicast group,
the switch starts an aging
timer for the port. When
the timer expires, the
dynamic member port
ages out.
IGMP membership
report.
The switch removes this
port from the IGMP
snooping forwarding
table.
NOTE:
In IGMP snooping, only dynamic ports age out. Static ports never age out.
How IGMP snooping works
In this section, the involved ports are dynamic ports. For information about how to configure and remove
static ports, see "
."
A switch that runs IGMP snooping performs different actions when it receives different IGMP messages.
When receiving a general query
The IGMP querier periodically sends IGMP general queries to all hosts and routers identified by the
address 224.0.0.1 on the local subnet to determine whether any active multicast group members exist on
the subnet.
After receiving an IGMP general query, the switch forwards it to all ports in the VLAN, except the port
that received the query. The switch also performs one of the following actions:
•
If the receiving port is a dynamic router port in the router port list, restarts the aging timer for the
port.
•
If the receiving port is not in its router port list, adds it into its router port list as a dynamic router port
and starts an aging timer for the port.
When receiving a membership report
A host sends an IGMP report to the IGMP querier for the following purposes:
•
If the host has been a member of a multicast group, responds to the query with an IGMP report.
•
Applies for joining a multicast group.
After receiving an IGMP report, the switch forwards it through all the router ports in the VLAN, resolves
the address of the reported multicast group. The switch also performs one of the following actions:
•
If no forwarding entry matches the group address, creates a forwarding entry for the group, adds
the receiving port as a dynamic member port to the forwarding entry, and starts an aging timer for
the port.
•
If a forwarding entry matches the group address, but the receiving port is not in the forwarding
entry for the group, adds the port as a dynamic member port to the forwarding entry and starts an
aging timer for the port.
•
If a forwarding entry matches the group address and the receiving port is in the forwarding entry
for the group, restarts the aging timer for the port.
A switch does not forward an IGMP report through a non-router port. If the switch forwards a report
message through a member port, the IGMP report suppression mechanism causes all the attached hosts
