The Internet Group Management Protocol (IGMP) is used by IP hosts to report their multicast group memberships to neighboring multicast devices. The IGMP Explicit Tracking feature enables a multicast device to explicitly track the membership of all multicast hosts in a particular multiaccess network. IGMP explicit tracking can be enabled globally and on Layer3 interfaces.

The explicit tracking of hosts, groups, and channels enables the device to keep track of each individual host that is joined to a particular group or channel. The main benefits of this feature are that it provides minimal leave latencies, faster channel changing, and improved diagnostics capabilities for IGMP.

The main benefit of the explicit tracking of hosts, groups, and channels in IGMP is to allow minimal leave latencies when a host leaves a multicast group or channel. The length of time between a host wanting to leave and a device stopping traffic forwarding is called the IGMP leave latency. A device configured with IGMP Version 3 (IGMPv3) and explicit tracking can immediately stop forwarding traffic if the last host to request to receive traffic from the device indicates that it no longer wants to receive traffic. The leave latency is thus bound only by the packet transmission latencies in the multiaccess network and the processing time in the device.

In IGMP Version 2, when a device receives an IGMP leave message from a host, it must first send an IGMP group-specific query to learn if other hosts on the same multiaccess network are still requesting to receive traffic. If after a specific time (the default value is approximately 3 seconds) no host replies to the query, the device will then stop forwarding the traffic. This query process is required because, in IGMP Version 1 and 2, IGMP membership reports are suppressed if the same report is already sent by another host in the network. Therefore, it is impossible for the device to reliably know how many hosts on a multiaccess network are requesting to receive traffic.

In networks where bandwidth is constrained between multicast devices and hosts (like in xDSL deployments), the bandwidth between devices and hosts is typically large enough to only sustain, in general, N multicast streams to be received in parallel. In these deployments, each host will typically join to only one multicast stream and the overall number of allowed hosts will be limited to N. The effective leave latency in these environments defines the channel change time of the receiver application—a single host cannot receive the new multicast stream before forwarding of the old stream has stopped. If an application tries to change the channel faster than the leave latency, the application will overload the bandwidth of the access network, resulting in a temporary degradation of traffic flow for all hosts. The explicit tracking of hosts, groups, and channels in IGMP allows for minimal leave latencies, and thus allows for fast channel changing capabilities.

The explicit tracking of hosts, groups, and channels in IGMP allows network administrators to easily determine which multicast hosts are joined to other multicast groups or channels.