H3C Technologies H3C S3100V2 Series Switches - Manual (Page 69)

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• If no forwarding table entry exists for the reported IPv6 multicast group, the switch creates an entry,
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, 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.