# 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:

- 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.

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 has been enabled on an IGMP V3 interface, even if a group has to downgrade to IGMP V2 due to the presence of an IGMP V2 host, explicit host tracking continues for that group. To avoid this, you can use the disable-if-igmp-v2-detected keyword. If you select this option, the router turns off explicit host tracking for the group when IGMP V2 host reports are received for the group on that interface. This option does not have any significance on an interface configured for IGMP V2 and is ignored if provided. Because IGMP V1 does not support leave messages, explicit host tracking is turned off for a group that downgrades to IGMP V1 due to the presence of IGMP V1 hosts.

Explicit host tracking cannot be enabled on an interface that has immediate-leave configured and vice versa. Any attempt to configure immediate-leave on an interface that has explicit host tracking enabled or to configure explicit host tracking on an interface that has immediate-leave enabled is rejected and an error message logged on the screen.

The following example enables IGMP V3 explicit host tracking on interface 3/0.101 with the default configuration where the router continues to perform explicit host tracking for IGMP V2 groups. To override this default configuration, you must use the ip igmp explicit-tracking disable-if-igmp-v2-detected command.

interface 3/0.101 ip igmp version 3 ip igmp explicit-tracking end

ip igmp explicit-tracking

- Use to set explicit host tracking for IP IGMP interfaces.
- To disable explicit host tracking if IGMP V2 hosts are detected, use the disable-if-igmp-v2-detected keyword.
- Example
  host1(config)#ip igmp explicit-tracking
- Use the no version to disable explicit host tracking on the interface. Use the no version with the disable-if-igmp-v2-detected keyword to revert to the default explicit host tracking behavior.
- See ip igmp explicit-tracking.