15
Timer Description
Expected message
before expiration
Action after
expiration
port ages out.
NOTE:
In IGMP snooping, only dynamic ports age out. Static ports never age out.
How IGMP snooping operates
An IGMP snooping-enabled switch performs different actions when it receives different IGMP
messages.
In this section, the involved ports are dynamic ports. For information about how to configure and
remove static ports, see "
."
When receiving a general query
To check for the existence of multicast group members, the IGMP querier periodically sends IGMP
general queries to all hosts and routers on the local subnet. All these hosts and routers are
indentified by the multicast address 224.0.0.1.
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 the router port list, adds it into the router port list as a dynamic
router port. It also 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 router ports in the VLAN. it also
resolves the address of the reported multicast group, and looks up the multicast forwarding table for
a matching entry:
•
If no match is found, the Layer 2 device creates a forwarding entry for the group with the
receiving port as an outing interface. It also marks the receiving port as a dynamic member port
and starts an aging timer for the port.
•
If a match is found but the receiving port is not in the forwarding entry, the Layer 2 device adds
the port as an outgoing interface to the entry. It also marks the port as a dynamic member port
and starts an aging timer for the port.
•
If a match is found and the receiving port is in the forwarding entry, the Layer 2 device restarts
the aging timer for the port.
A switch does not forward an IGMP report through a non-router port because of IGMP report
suppression mechanism. Assuming the switch forwards a report message through a member port,
all attached member receivers will receive the report and suppress their own reports. This makes the
switch unable to know whether the reported multicast group still has active members attached to that
port. For more information about the IGMP report suppression mechanism, see "
When receiving a leave message
An IGMPv1 host does not send any leave messages when it leaves a multicast group. Therefore, the
Layer 2 device cannot immediately update the status of the port that connects to the receiver host. In
this case, when the aging timer for the multicast group on the port expires, the Layer 2 device
