NOTE:
In IGMP snooping, only dynamic ports age out.
How IGMP snooping works
An IGMP snooping–enabled switch performs different actions when it receives different IGMP messages.
The ports in this section are dynamic 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 examine whether any active multicast group members exist on the subnet.
After receiving an IGMP general query, the switch forwards it through 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 the router port list, adds it into the 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 membership report to the IGMP querier for the following purposes:
If the host has been a member of a multicast group, responds to an IGMP query.
•
•
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 the following judgment:
If no forwarding entry matches the group address, the switch creates a forwarding entry for the
•
group, adds the port that received the IGMP report 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 port that received the IGMP report is not
•
in the forwarding entry for the group, the switch 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 port that received the IGMP report is in the
•
forwarding entry for the group, the switch 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 running on hosts causes all
attached hosts that monitor the reported multicast address to suppress their own reports. This makes the
switch unable to know whether the reported multicast group still has active members attached to that
port.
When receiving a leave group message
An IGMPv1 host silently leaves a multicast group and the switch is not notified of the leave. However,
because the host stops sending IGMP reports as soon as it leaves the multicast group, the switch removes
the port that connects to the host from the forwarding entry for the multicast group when the aging timer
for the port expires.
An IGMPv2 or IGMPv3 host sends an IGMP leave message to the multicast router when leaving a
multicast group.
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