Cisco Cisco IOS Software Release 12.2(14)S
IGMP Version 3—Explicit Tracking of Hosts, Groups, and Channels
Feature Overview
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Cisco IOS Release 12.2(14)S
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.
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.
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.
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.
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.
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.
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.