# Configuring Explicit Host Tracking

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 benefits:

- 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 option does not have any significance on an interface configured for MLD V1 and is ignored if selected.

If you execute the command on an interface that was previously enabled for immediate-leave, the configuration is accepted, immediate-leave is turned off and an appropriate warning message logged. Any attempt to configure immediate-leave on an interface that has explicit host tracking enabled is rejected and an error message logged.

The following example enables MLD V2 explicit host tracking on interface 3/0.101 with the default configuration where the router continues to perform explicit host tracking for MLD V1 groups. To override this default configuration, use the ip mld explicit-tracking disable-if-mld-v1-detected command.

interface 3/0.101 ip mld version 2 ip mld explicit-tracking end

ipv6 mld explicit-tracking

- Use to set explicit host tracking for MLD interfaces.
- To disable explicit host tracking if MLD V1 hosts are detected, use the disable-if-mld-detected keyword.
- Example
  host1(config)#ipv6 mld explicit-tracking
- Use the no version to disable explicit host tracking on the interface. Use the no version with the disable-if-mld-detected keyword to revert to the default explicit host tracking behavior.
- See ipv6 mld explicit-tracking.