DGS-3700-12/DGS-3700-12G Series Layer 2 Gigabit Ethernet Switch User Manual
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Multicast Listener Discovery (MLD) Snooping is an IPv6 function use d s im ilarl y to IGMP snooping in IPv4. It is used to
discover ports on a VLAN that are requesting multicast data. Instead of f looding all ports on a selected VLAN with
multicast traffic, MLD snooping will only forward multicast data to ports that wish to rec eive this data throug h the use
of queries and reports produced by the requesting ports and the source of the multicast traffic.
MLD snooping is accomplished through the examination of the layer 3 part of an MLD control packet t ransferred
between end nodes and a MLD router. When the Switch discovers that th is r ou te is r equ es tin g m ulticast traffic, it adds
the port directly attached to it into the correct IPv6 multicast table, and begins the process of for warding multicast
traffic to that port. This entry in the multicast routing table records the port, the VLAN ID and the ass ociated multic ast
IPv6 multicast group address and then considers this port to be a active l istening port. The ac tive listening ports are
the only ones to receive multicast group data.
MLD Control Messages
Three types of messages are transferred between devices using MLD snoop ing. These three messages are all
defined by three ICMPv6 packet headers, labeled 130, 131 and 132.
1. Multicast Listener Query – Similar to the IGMPv2 Host Membership Q uery for IPv4, and labeled as 130 in
the ICMPv6 packet header, this message is sent by the router to ask if any link is reques ting multicast data.
There are two types of MLD query messages emitted by the router. The G eneral Query is used to advertise all
multicast addresses that are ready to send multicast data to all list ening ports, and the Multicast Specific
query, which advertises a specific multicast address that is read y. These two types of messages are
distinguished by a multicast destination address located in the IPv6 head er and a multicast address in the
Multicast Listener Query Message.
2. Multicast Listener Report – Comparable to the Host Membership Report in IGMPv2, and labeled as 131 in
the ICMP packet header, this message is sent by the listening host to t he Switch statin g that it is inter ested in
receiving multicast data from a multicast address in response to the Mult icast Listener Query message.
3. Multicast Listener Done – Akin to the Leave Group Message in IGMPv2, and labeled as 132 i n the ICMP v6
packet header, this message is sent by the multicast listening host stating that it is no longer interested in
receiving multicast data from a specific multicast group address, therefor e stating that it is “done” with the
multicast data from this address. Once this message is received by the Switc h, it will no longer forward
multicast traffic from a specific multicast group address to this listening host.
MLD Snooping Settings
This table is used to enable MLD Snooping on the Switch and to configur e the settings for MLD snooping.
To view this window, click L2 Features > MLD Snooping > MLD Snooping Settings, as shown below:
Figure 3 - 41 MLD Snooping Settings window
To configure the settings for an existing entry click the correspo nding Edit button which will display the following
window.