306
NOTE:
In MLD snooping, only dynamic ports age out. Static ports never age out.
How MLD snooping operates
An MLD snooping–enabled switch performs different actions when it receives different MLD
messages.
In this section, the involved ports are dynamic ports. For information about how to configure and
remove static ports, see "
."
When receiving a general query
To check for the existence of IPv6 multicast group members, the MLD querier periodically sends
MLD general queries to all hosts and routers on the local subnet. All these hosts and routers are
identified by the address FF02::1.
After receiving an MLD general query, the switch forwards it to all ports in the VLAN, except the port
that received the query. The switch also performs one of the following actions:
•
If the receiving port is a dynamic router port in the router port list, restarts the aging timer for the
port.
•
If the receiving port is not in the router port list, adds it into the router port list as a dynamic
router port and starts an aging timer for the port.
When receiving a membership report
A host sends an MLD report to the MLD querier for the following purposes:
•
If the host has been a member of an IPv6 multicast group, responds to the query with an MLD
report.
•
Applies for joining an IPv6 multicast group.
After receiving an MLD report, the switch forwards it through all the router ports in the VLAN. It also
resolves the address of the reported IPv6 multicast group, and looks up the forwarding entry and
takes the following actions:
•
If no match is found, the switch creates a forwarding entry for the group with the receiving port
as an outgoing interface. It also marks the port as a dynamic member port and starts an aging
timer for the port.
•
If a match is found but the receiving port is not in the forwarding entry, the switch adds the port
as an outgoing interface to the entry. It also marks the port as a dynamic member port and starts
an aging timer for the port.
•
If a match is found and the receiving port is in the forwarding entry, the switch restarts the aging
timer for the port.
A switch does not forward an MLD report through a non-router port because of the MLD report
suppression mechanism. Assuming the switch forwards a report message through a member port,
the attached hosts that are members of the group will receive the report and suppress their own
reports. This makes the switch unable to know whether the reported IPv6 multicast group still has
active members attached to that port. For more information about the MLD report suppression
mechanism, see "
."
When receiving a done message
When a host leaves an IPv6 multicast group, the host sends an MLD done message to the multicast
routers. When the switch receives the MLD done message on a dynamic member port, the switch
first examines whether a forwarding entry matches the IPv6 multicast group address as follows:
•
If no match is found, the switch directly discards the MLD done message.
