D-Link DGS-3000-10TC Technical Information download pdf (Page 109)

DGS-3000 Series Layer 2 Managed Gigabit Switch Web UI Reference Guide

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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.

Chapter 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.

Chapter 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.

Chapter 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.

Chapter 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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D-Link DGS-3000-10TC Technical Information Page 109

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