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JunosE 11.3.x Multicast Routing Configuration Guide
Configuring Explicit Host Tracking
58
Explicit host tracking enables the router to explicitly track each individual host that is
joined to a group or channel on a particular multi-access network.
Explicit host tracking provides the following:
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, which 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 IGMP queries to verify membership.
As a result, interested hosts also do not need to respond to these queries with reports.
Tracking based on the IGMP 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 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 the group or channel.
If the host sending the leave message is the only host, it starts 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.
If one or more hosts that support only IGMP V1 are present on a network, the leave
latencies for the multicast groups to which those hosts are joined revert to the IGMP V1
leave latency. This affects only the multicast groups to which these legacy hosts are
actually joined at any point in time.
You cannot configure explicit host tracking on passive IGMP interfaces or on IGMP V1
interfaces. When you enable IGMP V2 or V3 on an interface, explicit host tracking is not
enabled by default.
Copyright © 2010, Juniper Networks, Inc.
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