3Com Switch 8800 Advanced Software V5 Configuration Guide

652 CHAPTER 45: MLD SNOOPING CONFIGURATION
■ Upon receiving an MLD query, an IPv6 multicast group member host responds
with an MLD report.
■ When intended to join an IPv6 multicast group, a host sends an MLD report to
the multicast router to announce that it is interested in the multicast
information addressed to that IPv6 multicast group.
Upon receiving an MLD report, the switch forwards it through all the router ports
in the VLAN, resolves the address of the IPv6 multicast group the host is interested
in, and performs the following to the receiving port:
■ If the port is already in the IPv6 forwarding table, the switch resets the member
port aging timer of the port.
■ If the port is not in the IPv6 forwarding table, the switch installs an entry for
this port in the IPv6 forwarding table and starts the member port aging timer
of this port.
n
A switch will not forward an MLD report through a non-router port for the
following reason: With MLD report suppression enabled, if member hosts of that
IPv6 multicast group still exist under other non-router ports, these hosts will stop
sending MLD reports when they receive the message. This prevents the switch
from knowing if members of that IPv6 multicast group are still attached to these
ports.
At present, the Switch 8800 supports only MLDv1 messages.
Done messages
When a host leaves an IPv6 multicast group, the host sends an MLD done message
to the multicast router to announce that it is to leave the IPv6 multicast group.
Upon receiving an MLD done message, a switch forwards it through all router
ports in the VLAN. Because the switch does not know whether any other member
hosts of that IPv6 multicast group still exists under the port to which the MLD
done message arrived, the switch does not immediately delete the forwarding
entry corresponding to that port from the forwarding table; instead, it resets the
aging timer of the member 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 group-specific query to that IPv6 multicast group through the port
that received the done message. Upon receiving the MLD group-specific query, a
switch forwards it through all the router ports in the VLAN and all member ports
of that IPv6 multicast group, and performs the following to the receiving port:
■ If an MLD report from that IPv6 multicast group arrives to this member port
before its aging timer expires as a response to the MLD group-specific query,
this means that some other members of that IPv6 multicast group still exist
under the port: the switch resets the aging timer of the member port.
■ If no MLD report from that IPv6 multicast group arrives to this member port
before its aging timer expires as a response to the MLD group-specific query,
this means that no members of that IPv6 multicast group still exist under the
member port: the switch deletes the forwarding entry for the member port
from the forwarding table when its aging timer expires.