Mld Snooping - D-Link DWS-3160-24TC Reference Manual

DWS-3160 Series Gigabit Ethernet Unified Switch Web UI Reference Guide

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.
4. Multicast Listener Report, Version 2 - Comparable to the Host Membership Report in IGMPv3, 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.
Data Driven Learning
The Switch allows you to implement data driven learning for MLD snooping groups. If data-driven learning, also
known as dynamic IP multicast learning, is enabled for a VLAN, when the Switch receives IP multicast traffic on the
VLAN, an MLD snooping group is created. Learning of an entry is not activated by MLD membership registration,
but activated by the traffic. For an ordinary MLD snooping entry, the MLD protocol will take care of the aging out of
the entry. For a data-driven entry, the entry can be specified not to age out or to age out by a timer.
When the data driven learning State is enabled, the multicast filtering mode for all ports is ignored. This means
multicast packets will be flooded.
NOTE: If a data-driven group is created and MLD member ports are learned later, the entry will
become an ordinary MLD snooping entry. In other words, the aging out mechanism will follow
the conditions of an ordinary MLD snooping entry.
Data driven learning is useful on a network which has video cameras connected to a Layer 2 switch that is
recording and sending IP multicast data. The switch needs to forward IP data to a data centre without dropping or
flooding any packets. Since video cameras do not have the capability to run MLD protocols, the IP multicast data
will be dropped with the original MLD snooping function.
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