IGMP Explicit Tracking
This module describes the explicit tracking of hosts, groups, and channels for the Internet Group Management Protocol (IGMP).
Restrictions for IGMP Explicit Tracking
The following restrictions apply to this feature.
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If one or more hosts that supports only IGMP Version 1 or Version 2 are present on a network, the leave latencies for the multicast groups to which these hosts are joined will revert to the leave latencies of the IGMP version of the hosts—approximately 3 seconds for IGMP Version 2 and up to 180 seconds for IGMP Version 1. This condition affects only multicast groups to which these legacy hosts are actually joined at any given point in time. In addition, the membership reports for these multicast groups sent by IGMPv3 hosts may revert to IGMP Version 1 or Version 2 membership reports, thus disabling explicit tracking of those host memberships.
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Explicit tracking of IGMP Version 3 lite (IGMP v3lite) or URL Rendezvous Directory (URD) channel membership reports is not supported. Therefore, the leave latency for multicast groups sending traffic to hosts using IGMPv3 lite or URD will be determined by the leave latency of the version of IGMP configured on the hosts (for IGMPv3, the leave latency is typically 3 seconds when explicit tracking is not configured).
Information About IGMP Explicit Tracking
IGMP Explicit Tracking
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.
Minimal Leave Latencies
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.
Faster Channel Changing
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.
Improved Diagnostic 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.
How to Configure IGMP Explicit Tracking
Enabling Explicit Tracking Globally
Procedure
Command or Action | Purpose | |
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Step 1 |
enable Example:
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Enables privileged EXEC mode.
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Step 2 |
configure terminal Example:
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Enters global configuration mode. |
Step 3 |
ip igmp snooping vlan vlan-id explicit-tracking Example:
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Enables IGMP explicit host tracking. |
Step 4 |
exit Example:
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Exits global configuration mode and returns to privileged EXEC mode. |
Enabling Explicit Tracking on Layer 3 Interfaces
Procedure
Command or Action | Purpose | |
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Step 1 |
enable Example:
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Enables privileged EXEC mode.
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Step 2 |
configure terminal Example:
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Enters global configuration mode. |
Step 3 |
interface type number Example:
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Configures an interface and enters interface configuration mode. |
Step 4 |
ip address ip-address mask Example:
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Sets a primary or secondary IP address for an interface. |
Step 5 |
ip pim sparse-mode Example:
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Enables Protocol Independent Multicast (PIM) sparse mode on an interface. |
Step 6 |
ip igmp version 3 Example:
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Configure Internet Group Management Protocol (IGMP) Version 3 (IGMPv3) on the device. |
Step 7 |
ip igmp explicit-tracking Example:
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Enables IGMP explicit host tracking. |
Step 8 |
exit Example:
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Exits global configuration mode and returns to privileged EXEC mode. |
Configuration Examples for IGMP Explicit Tracking
Example: Enabling Explicit Tracking
The following example shows a basic configuration to enable IGMP explicit tracking globally:
Device# configure terminal
Device(config)# ip multicast routing
Device(config)# ip igmp snooping vlan 1 explicit-tracking
Device(config)# end
The following example shows a basic configuration to enable IGMP explicit tracking on Layer 3 interfaces:
Device# configure terminal
Device(config)# interface vlan 77
Device(config-if)# ip address 10.1.1.1 255.255.255.254
Device(config-if)# ip pim sparse-mode
Device(config-if)# ip igmp version 3
Device(config-if)# ip igmp explicit-tracking
Device(config-if)# end
Displaying IGMP Explicit Tracking Information
To display host membership information, perform this task:
Procedure
Step 1 |
enable Example:
Enables privileged EXEC mode. Enter your password, if prompted. |
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Step 2 |
show ip igmp snooping membership [ interface interface_num ] [ vlan vlan-id ] [ reporter a.b.c.d ] [ source a.b.c.d group a.b.c.d ] Example:
Displays Explicit Host Tracking (EHT) information. This command is valid only if EHT is enabled on the switch.
With the EHT feature enabled, the entries that are updated in the IGMP Snooping Membership table do not age out. Use the clear ip igmp snooping membership vlan command to clear the entries from the explicit host tracking table. |
Example
The following example shows how to display host membership information for VLAN 100 and to delete the EHT database:
Device# show ip igmp snooping membership vlan 100
Snooping Membership Summary for Vlan 100
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Total number of channels: 2
Total number of hosts : 1
Source/Group Interface Reporter Vlan Uptime Last-Join/Last-Leave
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0.0.0.0/228.1.1.1 Po9 99.99.1.2 100 00:00:00 00:00:01/00:00:01
0.0.0.0/228.1.1.2 Po9 99.99.1.2 100 00:00:00 00:00:01/00:00:01
Device# clear ip igmp snooping membership vlan 100
Verifying IGMP Explicit Tracking
Procedure
Step 1 |
enable Example:
Enables privileged EXEC mode.
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Step 2 |
show ip igmp snooping vlan vlan-ID Example:
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Step 3 |
show ip igmp groups interface-type interface-number Example:
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Step 4 |
show ip igmp membership tracked Example:
Displays the multicast groups with the explicit tracking feature enabled.
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Step 5 |
show ip igmp snooping vlan vlan-ID Example:
Displays the IGMP snooping configuration on a VLAN.
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Feature History for IGMP Explicit Tracking
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Feature Name |
Release |
Feature Information |
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IGMP Explicit Tracking |
Cisco IOS XE Everest 16.6.1 |
This module describes the explicit tracking of hosts, groups, and channels for IGMP. |