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•
If a forwarding table entry exists for the reported IPv6 multicast group, but the port is not
included in the outgoing port list for that group, the switch adds the port as a dynamic member
port to the outgoing port list, and starts a member port aging timer for that port.
•
If a forwarding table entry exists for the reported IPv6 multicast group and the port is included in
the outgoing port list, which means that this port is already a dynamic member port, the switch
resets the member port aging timer for that port.
NOTE:
A switch does not forward an MLD report through a non-router port. This is because if the switch
forwards a report message through a member port, all the attached hosts listening to the reported
IPv6 multicast address will suppress their own reports upon receiving this report according to the
MLD report suppression mechanism for hosts, and this will prevent the switch from knowing
whether the reported multicast group still has active members attached to that port.
Done messages
When a host leaves an IPv6 multicast group, the host sends an MLD done message to the multicast
router.
When the switch receives an MLD done message on a dynamic member port, the switch first
determines whether a forwarding table entry for the IPv6 multicast group address in the message
exists, and, if one exists, whether the outgoing port list contains the port.
•
If the forwarding table entry does not exist or if the outgoing port list does not contain the port,
the switch discards the MLD done message instead of forwarding it to any port.
•
If the forwarding table entry exists and the outgoing port list contains the port, the switch
forwards the MLD done message to all router ports in the native VLAN. Because the switch has
not determined whether any other hosts attached to the port are still monitoring that IPv6
multicast group address, the switch does not immediately remove the port from the outgoing
port list of the forwarding table entry for that group. Instead, it resets the aging timer for the port.
After receiving an MLD done message from a host, the MLD querier resolves the IPv6 multicast
group address in the message and sends an MLD multicast-address-specific query to that IPv6
multicast group address through the port that received the MLD done message. After receiving the
MLD multicast-address-specific query, the switch forwards it through all the router ports in the VLAN
and all member ports for that IPv6 multicast group, and performs the following to the receiving port:
•
If any MLD report in response to the MLD multicast-address-specific query is received on the
port—suppose it is a dynamic member port—before its aging timer expires, this means that
some host attached to the port is receiving or expecting to receive IPv6 multicast data for that
IPv6 multicast group. The switch resets the aging timer for the port.
•
If no MLD report in response to the MLD multicast-address-specific query is received on the
port before its aging timer expires, this means that no hosts attached to the port are still
monitoring that IPv6 multicast group address. The switch removes the port from the outgoing
port list of the forwarding table entry for that IPv6 multicast group when the aging timer expires.
MLD snooping proxying
You can configure the MLD snooping proxying function on an edge device to reduce the number of
MLD reports and done messages sent to its upstream device. The device configured with MLD
snooping proxying is called an MLD snooping proxy. It is a host from the perspective of its upstream
device.
NOTE:
Even though an MLD snooping proxy is a host from the perspective of its upstream device, the MLD
membership report suppression mechanism for hosts does not take effect on it.