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xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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Figure 7- 25. IGMP Multicast VLAN Entries window
To configure the IGMP Snooping Multicast VLAN settings, click its corresponding
button, which will produce the
following window for the user to configure. To delete an entry, click the corresponding
under the Delete heading.
Figure 7- 26. IGMP Multicast VLAN - Modify window
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 a 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 three
ICMPv6 packet headers, labeled 130, 131 and 132.
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 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.