9.2
MLD Snooping
MLD Snooping
Multicast Listener Discovery (MLD) snooping is applied for efficient distribution of IPv6
multicast data to clients and routers in a Layer 2 network. With MLD snooping, IPv6 multicast
data is selectively forwarded to a list of ports that want to receive the data, instead of being
flooded to all ports in a VLAN. The list is constructed and maintained by snooping IPv6
multicast control packets. MLD snooping performs a similar function in IPv6 as IGMP snooping
in IPv4.
The switch, running MLD Snooping, listens to the MLD messages transmitted between the host
and the router, and tracks the MLD messages and the registered port. When receiving MLD
report message, the switch adds the port to the multicast address table; when the switch
listens to MLD Done message from the host, the router sends the Multicast-Address-Specific
Query message of the port to check if other hosts need this multicast, if yes, the switch will
receive MLD report message; if no, the switch will receive no response from the hosts and the
switch will remove the port from the multicast address table. The router regularly sends MLD
query messages. After receiving the MLD query messages, the switch will remove the port
from the multicast address table if the switch receives no MLD report message from the host
within a period of time.
MLD Snooping Fundamentals
1.
MLD Messages
MLD Queries
：
MLD Queries include General Queries and Multicast-Address-Specific Queries
(MASQs) and are sent out from the MLD
router.
MLD Reports
：
When a host wants to join a multicast group or responds to the MLD queries, it
will send out an MLD report.
MLD Done Messages
：
When a host wants to leave a multicast group, it will send out an MLD
Done message to inform the IPv6 multicast routers of its leave.
2.
Relevant Ports of the Switch
Router Port:
Indicates the switch port that links toward the MLD router.
Member Port:
Indicates the switch port that links toward the multicast members.
3.
Timers
Router Port Aging Time:
Within this time, if the switch does not receive MLD queries from the
router port, it will delete this port from the router port list. The default value is 300 seconds.
Member Port Aging Time:
Within this time, if the switch does not receive MLD reports from the
member port, it will delete this port from the MLD multicast group. The default value is 260
seconds.
General Query Interval:
The interval between the multicast router sends out general queries.
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