H3C Technologies H3C S10500 Series Switches - Manual (Page 274)

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• 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. The reason is that if the switch
forwards a report message through a member port, all the attached hosts that monitor the reported IPv6
multicast address suppress their own reports after receiving this report according to the MLD report
suppression mechanism. This prevents the switch from determining whether the reported multicast group
still has active members attached to that port.
• For more information about the MLD report suppression mechanism of hosts, see the chapter “MLD
configuration.”
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 checks
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 does not know
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. The switch also performs the following judgment on the port
that received the MLD done message:
• If the port (a dynamic member port supposed) receives any MLD report in response to the MLD
multicast-address-specific query before its aging timer expires, it indicates that a host attached to
the port is receiving or expecting to receive IPv6 multicast data destined for that IPv6 multicast
group. The switch resets the aging timer for the port.
• If the port does not receive an MLD report in response to the MLD multicast-address-specific query
before its aging timer expires, it indicates 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.