H3C S5500-EI Series Operation Manual page 57

Operation Manual – Multicast
H3C S5500-EI Series Ethernet Switches
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 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 as
follows: Due to the MLD report suppression mechanism, 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 hearing this report, and this will
prevent the switch from knowing whether any hosts attached to that port are still active
members of the reported IPv6 multicast group.
III. 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 a group-specific MLD done message on a member port, it
first checks whether a forwarding table entry for that IPv6 multicast group exists, and, if
one exists, whether its outgoing port list contains that port.
If the forwarding table entry does not exist or if its 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 its outgoing port list contains the port, the
switch forwards the done message to all router ports in the VLAN. Because the
switch does not know whether any other hosts attached to the port are still
listening to that IPv6 multicast group address, the switch does not immediately
removes the port from the outgoing port list of the forwarding table entry for that
group; instead, it resets the member port aging timer for the port.
Upon receiving an MLD done message from a host, the MLD querier resolves from the
message the address of the IPv6 multicast group that the host just left and sends an
MLD multicast-address-specific query to that IPv6 multicast group through the port that
received the done message. Upon hearing the MLD multicast-address-specific query,
the switch forwards it through all its 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
heard on a 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 of the member port.
Chapter 3 MLD Snooping Configuration
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