Mld Snooping - D-Link Dis-200G Series Web Ui Reference Manual

MLD Snooping

Multicast Listener Discovery (MLD) Snooping is an IPv6 function used similarly to IGMP snooping in IPv4.
It is used to discover ports on a VLAN that are requesting multicast data. Instead of flooding all ports on a
selected VLAN with multicast traffic, MLD snooping will only forward multicast data to ports
that wish to receive this data through 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
transferred between end nodes and a MLD router. When the Switch discovers that this route is requesting
multicast traffic, it adds the port directly attached to it into the correct IPv6 multicast table, and begins the
process of forwarding multicast traffic to that port. This entry in the multicast routing table records the
port, the VLAN ID, and the associated multicast IPv6 multicast group address, and then considers this
port to be an active listening port. The active listening ports are the only ones to receive multicast group
data.
MLD Control Messages
Three types of messages are transferred between devices using MLD snooping. These three messages
are all defined by four ICMPv6 packet headers, labeled 130, 131, 132, and 143.
1. Multicast Listener Query – Similar to the IGMPv2 Host Membership Query for IPv4, and labeled
as 130 in the ICMPv6 packet header, this message is sent by the router to ask if any link is
requesting multicast data. There are two types of MLD query messages emitted by the router. The
General Query is used to advertise all multicast addresses that are ready to send multicast data to
all listening ports, and the Multicast Specific query, which advertises a specific multicast address
that is also ready. These two types of messages are distinguished by a multicast destination
address located in the IPv6 header and a multicast address in the Multicast Listener Query
Message.
2. Multicast Listener Report, Version 1 – Comparable to the Host Membership Report in IGMPv2,
and labeled as 131 in the ICMP packet header, this message is sent by the listening port to the
Switch stating that it is interested in receiving multicast data from a multicast address in response to
the Multicast Listener Query message.
3. Multicast Listener Done – Akin to the Leave Group Message in IGMPv2, and labeled as 132 in the
ICMPv6 packet header, this message is sent by the multicast listening port stating that it is no longer
interested in receiving multicast data from a specific multicast group address, therefore stating that it
is "done" with the multicast data from this address. Once this message is received by the Switch, it
will no longer forward multicast traffic from a specific multicast group address to this listening port.
Multicast Listener Report, Version 2 – Comparable to the Host Membership Report in IGMPv3,
4.
and labeled as 143 in the ICMP packet header, this message is sent by the listening port to the
Switch stating that it is interested in receiving multicast data from a multicast address in response to
the Multicast Listener Query message.
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