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Table Of Contents
IGMP Version 3—Explicit Tracking of Hosts, Groups, and Channels
Supported Standards, MIBs, and RFCs
IGMP Version 3—Explicit Tracking of Hosts, Groups, and Channels
Feature History
This document describes the IGMP Version 3—Explicit Tracking of Hosts, Groups, and Channels feature in Cisco IOS Release 12.2(14)S and includes the following sections:
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Supported Standards, MIBs, and RFCs
Feature Overview
The Internet Group Management Protocol (IGMP) is used by IP hosts to report their multicast group memberships to neighboring multicast routers. IGMP is available in versions 1, 2, and 3. IGMP Version 3 (IGMPv3) is supported in Cisco IOS Release 12.0(15)S, 12.1(5)T, 12.1(8)E, and later releases.
The IGMP Version 3—Explicit Tracking of Hosts, Groups, and Channels feature enables a multicast router to explicitly track the membership of all multicast hosts in a particular multiaccess network. This enhancement to the Cisco IOS implementation of IGMPv3 enables the router to track 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.
For more information on IGMPv3 and related features such as Source Specific Multicast (SSM), refer to the Cisco IOS IP Configuration Guide, Release 12.2.
Benefits
Minimal Leave Latencies
The main benefit of the IGMP Version 3—Explicit Tracking of Hosts, Groups, and Channels feature is to allow minimal leave latencies when a host leaves a multicast group or channel. A router configured with IGMPv3 and explicit tracking can immediately stop forwarding traffic if the last host to request to receive traffic from the router 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 router.
In IGMP Version 2, when a router 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 (in Cisco IOS software, the default value is approximately 3 seconds) no host replies to the query, the router 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 has already been sent by another host in the network. Therefore, it is impossible for the router 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 routers and hosts (like in xDSL deployments), the bandwidth between routers and hosts is typically large enough to only sustain, in general, x 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 x. 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 IGMP Version 3—Explicit Tracking of Hosts, Groups, and Channels feature allows for minimal leave latencies, and thus allows for fast channel changing capabilities.
Improved Diagnostics Capabilities
The IGMP Version 3—Explicit Tracking of Hosts, Groups, and Channels feature allows network administrators to easily determine which multicast hosts are joined to which multicast groups or channels.
Restrictions
No MIB Support
There is no Simple Network Management Protocol (SNMP) MIB to track the IGMP membership of individual hosts. The MIBs supported by Cisco IOS software reflect only the aggregate membership of a particular interface on a router.
No Minimal Leave Latency for Groups with Legacy Hosts
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 those 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 the multicast groups to which those 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.
No Explicit Tracking Support for IGMP v3lite and URD
Explicit tracking of IGMP Version 3 lite (IGMP v3lite) or URL Rendezvous Directory (URD) channel membership reports is not supported in Release 12.0(19)S or earlier releases. In these releases, the leave latency for multicast groups sending traffic to hosts using IGMP v3lite 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).
Related Documents
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Supported Platforms
This feature runs on all platforms that support Cisco IOS Release 12.2(14)S, Release 12.2(8)T, Release 12.0(19)S, or later releases.
Determining Platform Support Through Cisco Feature Navigator
Cisco IOS software is packaged in feature sets that are supported on specific platforms. To get updated information regarding platform support for this feature, access Cisco Feature Navigator. Cisco Feature Navigator dynamically updates the list of supported platforms as new platform support is added for the feature.
Cisco Feature Navigator is a web-based tool that enables you to determine which Cisco IOS software images support a specific set of features and which features are supported in a specific Cisco IOS image. You can search by feature or release. Under the release section, you can compare releases side by side to display both the features unique to each software release and the features in common.
To access Cisco Feature Navigator, you must have an account on Cisco.com. If you have forgotten or lost your account information, send a blank e-mail to cco-locksmith@cisco.com. An automatic check will verify that your e-mail address is registered with Cisco.com. If the check is successful, account details with a new random password will be e-mailed to you. Qualified users can establish an account on Cisco.com by following the directions found at this URL:
Cisco Feature Navigator is updated regularly when major Cisco IOS software releases and technology releases occur. For the most current information, go to the Cisco Feature Navigator home page at the following URL:
Availability of Cisco IOS Software Images
Platform support for particular Cisco IOS software releases is dependent on the availability of the software images for those platforms. Software images for some platforms may be deferred, delayed, or changed without prior notice. For updated information about platform support and availability of software images for each Cisco IOS software release, refer to the online release notes or, if supported, Cisco Feature Navigator.
Supported Standards, MIBs, and RFCs
Standards
No new or modified standards are supported by this feature.
MIBs
No new or modified MIBs are supported by this feature.
To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:
http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml
RFCs
No new or modified RFCs are supported by this feature.
Configuration Tasks
See the following sections for configuration tasks for this feature. Each task in the list is identified as either required or optional.
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Enabling Explicit Tracking
To enable explicit tracking on an interface, use the following commands in interface configuration mode:
Verifying Explicit Tracking
To verify configuration of explicit tracking, use the following show commands in EXEC mode:
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Note
Configuration Examples
The following example shows a basic configuration for enabling IP multicast with SSM, IGMPv3, and explicit tracking:
ip multicast routinginterface ethernet 0description access network to desktop systemsip address 10.1.0.1 255.255.255.0ip pim sparse-dense-modeip mroute-cacheip igmp version 3ip igmp explicit-trackingip igmp v3liteip urdinterface ethernet 1description backbone interface !No hosts connectedip address 10.10.0.1 255.255.255.0ip pim sparse-dense-modeip mroute-cacheip pim ssm defaultCommand Reference
This section documents new commands. All other commands used with this feature are documented in the Cisco IOS Release 12.2 command reference publications.
ip igmp explicit-tracking
To enable explicit tracking of hosts, groups, and channels for IGMP Version 3 (IGMPv3), use the ip igmp explicit-tracking command in interface configuration mode. To disable this feature, use the no form of this command.
ip igmp explicit-tracking
no ip igmp explicit-tracking
Syntax Description
This command has no arguments or keywords.
Defaults
Disabled
Command Modes
Interface configuration
Command History
Usage Guidelines
This command can be configured on an interface only if IGMPv3 is first configured on the same interface. To configure IGMPv3, use the ip igmp version 3 command in interface configuration mode.
When explicit tracking is enabled on a router, the router can individually track the Internet Group Management Protocol (IGMP) membership state of all reporting hosts. This feature allows the router to achieve minimal leave latencies when hosts leave a multicast group or channel. To monitor IGMP membership of hosts, use the show ip igmp membership command in EXEC mode.
When explicit tracking is enabled, the router uses more memory than if explicit tracking is disabled because the router must store the membership state of all hosts on the interface.
Examples
The following example shows a basic configuration for enabling IP multicast with Source Specific Multicast (SSM), IGMPv3, and explicit tracking:
ip multicast routinginterface ethernet 0description access network to desktop systemsip address 10.1.0.1 255.255.255.0ip pim sparse-dense-modeip mroute-cacheip igmp version 3ip igmp explicit-trackingip igmp v3liteip urdinterface ethernet 1description backbone interface !No hosts connectedip address 10.10.0.1 255.255.255.0ip pim sparse-dense-modeip mroute-cacheip pim ssm defaultRelated Commands
ip igmp version
Configures the version of IGMP that the router uses.
Displays the IGMP membership information for multicast groups and (S, G) channels.
show ip igmp membership
To display Internet Group Management Protocol (IGMP) membership information for multicast groups and (S, G) channels, use the show ip igmp membership command in EXEC mode.
show ip igmp membership [group-address | group-name] [tracked] [all]
Syntax Description
Command Modes
EXEC
Command History
Usage Guidelines
Unlike the show ip igmp groups command, this command allows you to display detailed information about multicast channels and explicit tracking.
Examples
The following is sample output from the show ip igmp membership command. Each entry in the output shows the aggregate membership information (indicated by the A flag) for a particular multicast group or channel from the IGMP cache. If the entry is prepended with a forward slash ("/") flag, the entry is a filtering entry that is blocking the data forwarding of the multicast group or channel.
Router> show ip igmp membershipFlags:A - aggregate, T - trackedL - Local, S - static, V - virtual, R - Reported through v3I - v3lite, D - Urd, M - SSM (S,G) channel1,2,3 - The version of IGMP, the group is inChannel/Group-Flags:/ - Filtering entry (Exclude mode (S,G), Include mode (*,G))Reporter:<ip-address> - last reporter if group is not explicitly tracked<n>/<m> - <n> reporter in include mode,<m> reporter in excludeChannel/Group Reporter Uptime Exp. Flags Interface*,224.0.1.40 10.10.0.1 00:01:34 02:41 2LA Et2/0*,239.1.1.1 2/0 00:00:10 stop 3AT Et2/0The following is sample output from the show ip igmp membership command with the multicast group address 239.1.1.1 and the tracked keyword specified:
Router> show ip igmp membership 239.1.1.1 trackedFlags:A - aggregate, T - trackedL - Local, S - static, V - virtual, R - Reported through v3I - v3lite, D - Urd, M - SSM (S,G) channel1,2,3 - The version of IGMP, the group is inChannel/Group-Flags:/ - Filtering entry (Exclude mode (S,G), Include mode (*,G))Reporter:<ip-address> - last reporter if group is not explicitly tracked<n>/<m> - <n> reporter in include mode,<m> reporter in excludeChannel/Group Reporter Uptime Exp. Flags Interface*,239.1.1.1 2/0 00:00:11 stop 3AT Et2/010.30.0.100,239.1.1.1 10.10.0.10 00:00:11 02:48 RT Et2/010.30.0.101,239.1.1.1 10.10.0.20 00:00:03 02:56 RT Et2/010.30.0.101,239.1.1.1 10.10.0.10 00:00:11 02:48 RT Et2/010.30.0.102,239.1.1.1 10.10.0.20 00:00:03 02:56 RT Et2/0Table 1 describes the significant fields shown in the displays.
Related Commands
