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:

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:

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.

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