Chapter 12
MLD Configuration
Table of Contents
12.1 MLD Overview
MLD originates from IGMP. MLDv1 corresponds to IGMPv2, and MLDv2 corresponds to
IGMPv3. Different from IGMP which uses the message type of IP protocol number 2, MLD
uses the message type of ICMPv6 (the IP protocol number is 58), including MLD Query
message (type value is 130, MLDv1 Report message (type value is 131), MLDv1 Leaving
message (type value is 132) and MLDv2 Report message (type value is 143). MLD acts
completely the same as IGMP does except for the message structure.
MLD provides the information that is needed in the last state to forward a multicast packet
to the destination. The multicast router exchanges information with the host receiving the
multicast data. The information is collected from the group members of hosts that connect
to the multicast router directly.
MLD mainly uses two types of messages, group member query message and group
member report message.
l
The multicast router sends group member query messages to all hosts periodically to
determine the group members on the interconnected subnets.
l
Each host replies with a member report message to report the multicast group which
it belongs to.
When a host joins a new group, it sends a join message immediately instead of waiting
for a query in case it is the first member of the group.
When a host begins to receive information as a member of a group, the multicast router
will query the group periodically to check whether there is any member in the group. As
long as there is one host, the multicast router continues forwarding data.
When a host leaves a group, the multicast router receives a leaving message, and then
it queries whether there is any other active member in the group immediately. If there is
any, the multicast router continues forwarding data. Otherwise, it will not forward data any
longer.
At present, there are two versions of MLD. MLDv1 corresponds to IGMPv2, and it provides
the fast leaving mechanism of group members. MLDv2 corresponds to IGMPv3, and
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