Juniper JUNOSE 11.0.X MULTICAST ROUTING Configuration Manual page 219

Minimal leave latency when a host leaves a multicast group or channel. When
the router receives a leave message for a group or channel on an interface, it
accesses a list of hosts and immediately stops forwarding traffic if the sender is
the last host to request traffic for that group or channel. The leave latency is
bound only by the packet transmission latencies in the multi-access network
and the processing time in the router.
Ability to change channels quickly in networks where bandwidth is constrained
between a multicast-enabled router and hosts.
Ability to determine what multicast hosts are joined to particular multicast groups
or channels; this is useful for accounting purposes.
Reduction of control message traffic on the network because, when it receives
a leave message, the router no longer needs to send out MLD queries to verify
membership. As a result, interested hosts also do not need to respond to these
queries with reports.
Tracking based on MLD reports for hosts in both include and exclude modes for
every multicast group or channel on an interface.
When the router is configured for explicit host tracking and starts performing
immediate leave using the host information collected, every leave message received
for a group or channel is treated as follows:
The router checks the number of hosts that receive traffic from this group or
channel.
If the host sending the leave message is the only host, it performs immediate
leave for that group or channel on that interface. The router removes the interface
from the multicast group or channel immediately, without sending out a group
or group-source specific query and waiting for the last member query interval.
If the host sending the leave message is not the only host receiving traffic for
that group or channel, the router removes the host from the list of hosts on that
interface, but keeps the interface in the outgoing interface list for the multicast
group or channel. No group or group-source specific queries are sent.
You can enable MLD explicit host tracking on an interface only if MLD V1 or V2 has
been previously enabled on the interface. Explicit host tracking is not enabled by
default when you enable MLD on the interface. Explicit host tracking cannot be
configured on passive MLD interfaces.
When you enable explicit host tracking on an interface that has a membership state,
the router does not immediately start performing immediate leave. For a maximum
of group membership interval seconds, the router only performs host tracking. Any
leave messages that the router receives during this period receive normal leave
processing. Any leave messages received after this interval has elapsed receive
immediate leave processing, when appropriate.
When explicit host tracking is enabled on an MLD V2 interface, even if a group has
to downgrade to MLD V1 due to the presence of an MLD V1 host, explicit host tracking
continues for that group. To avoid this, you can use the disable-if-mld-v1-detected
keyword. If you select this option, the router turns off explicit host tracking for the
group when MLD V1 host reports are received for the group on that interface. This
Chapter 6: Configuring Multicast Listener Discovery
Configuring Explicit Host Tracking
197