137
Timer Description
Message before
expiration
Action after expiration
Dynamic member port
aging timer
When a port dynamically
joins a multicast group,
the switch starts or resets
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
An IGMP snooping-enabled router performs different actions when it receives different IGMP messages.
The ports in this section are dynamic ports. For information about how to configure and remove static
ports, see "
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 determine whether any active multicast group members exist on the subnet.
After receiving an IGMP general query, the IGMP snooping-enabled router forwards it to all ports in the
VLAN, except the port that received the query. This router also performs one of the following actions:
•
If the port that received the query is a dynamic router port in the router port list, restarts the aging
timer for the port.
•
If the port that received the query 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 report to the IGMP querier for the following purposes:
•
Responds to queries if the host is a multicast group member.
•
Applies for joining a multicast group.
After receiving an IGMP report, the IGMP snooping-enabled router forwards it through all the router
ports in the VLAN, resolves the address of the reported multicast group, and performs the following
judgment:
•
If no forwarding entry matches the group address, 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, 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, restarts the aging timer for the port.
An IGMP snooping-enabled router does not forward an IGMP report through a non-router port. If the
IGMP snooping-enabled router forwards a report message through a member port, the IGMP report
