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DES-7200 Configuration Guide Chapter 4 MLD
Configuration
4-2
the multicast listener state table will then be activated.
Currently, MLD has to versions. MLDv2 was developed on the basis of MLDv1 by
adding the source filtering mechanism. The behaviors of MLD protocol can be
divided into two parts: host behavior and router behavior.
4.1.1 Introduction to Messages of Different MLD versions
4.1.1.1 MLD Version 1
In MLDv1, there are three types of messages:
Multicast Listener Query
Multicast Listener Report
Multicast Listener Done
In the multicast network running MLD protocol, there will be querier responsible
for sending MLD query messages. Such querier is determined through election.
In the beginning, all devices are of the querier state. When the devices receives
multicast listener query from a device with a lower IP address, they will change
from querier state to non-querier state. Therefore, only one device will be of the
querier state, and this device has the lowest IP address among all multicast
devices on the network.
MLDv1 also has the corresponding mechanism to handle the failure of querier
device. Non-querier devices will maintain the current interval timer of other
queriers. The device will reset this timer when every time it receives the multicast
listener query message. If this timer times out, then the corresponding device will
restart sending query message, and the new round of querier device election will
begin again.
The querier device must periodically send out the multicast listener query to
ensure that non-querier devices on the network know that the querier device is
still workable. In realize this function, the querier device maintains a query interval
timer, which will be reset when the multicast listener query message is sent out.
When the query interval timer is set to zero or no longer useful, the querier device
will send out another multicast listener query.
When MLD protocol is initiated, it will send a number of general query messages
to discover which multicast groups shall be forwarded on the specific interface.
The number of ordinary query messages sent by the device will be based on the
Startup Query Count configured by the current device. The space between
startup general queries will be determined by the value of Startup Query
Interval. 。
When the querier device receives the leave message, it must send a
group-specific multicast listener query message to see whether the host is the
last listener to leave the group. The device will send a number of group-specific
query messages before it stops forwarding data messages for the specific group,
and the number of messages equals to the number of last listener queries. The
device will send multiple group-specific multicast listener queries to ensure there
is no more listener in that group. A group-specific query will be sent at every other
last listener query interval in order to partition the queries. When no response is
received, the device will delete the corresponding group record, and stop
forwarding multicast data messages on that specific interface for this group
address.