IGMPv3 can operate with both Internet Standard Multicast (ISM) and Source Specific Multicast (SSM). In
ISM, both exclude and include mode reports are applicable. In SSM, only include mode reports are accepted
by the last-hop router. Exclude mode reports are ignored.
IGMP Join and Leave Process
IGMP Join Process
When a host wants to join a multicast group, the host sends one or more unsolicited membership reports for
the multicast group it wants to join. The IGMP join process is the same for IGMPv1 and IGMPv2 hosts.
In IGMPv3, the join process for hosts proceeds as follows:
•
When a hosts wants to join a group, it sends an IGMPv3 membership report to 224.0.0.22 with an empty
EXCLUDE list.
•
When a host wants to join a specific channel, it sends an IGMPv3 membership report to 224.0.0.22 with
the address of the specific source included in the INCLUDE list.
•
When a host wants to join a group excluding particular sources, it sends an IGMPv3 membership report
to 224.0.0.22 excluding those sources in the EXCLUDE list.
If some IGMPv3 hosts on a LAN wish to exclude a source and others wish to include the source, then the
device will send traffic for the source on the LAN (that is, inclusion trumps exclusion in this situation).
Note
IGMP Leave Process
The method that hosts use to leave a group varies depending on the version of IGMP in operation.
IGMPv1 Leave Process
There is no leave-group message in IGMPv1 to notify the devices on the subnet that a host no longer wants
to receive the multicast traffic from a specific group. The host simply stops processing traffic for the multicast
group and ceases responding to IGMP queries with IGMP membership reports for the group. As a result, the
only way IGMPv1 devices know that there are no longer any active receivers for a particular multicast group
on a subnet is when the devices stop receiving membership reports. To facilitate this process, IGMPv1 devices
associate a countdown timer with an IGMP group on a subnet. When a membership report is received for the
group on the subnet, the timer is reset. For IGMPv1 devices, this timeout interval is typically three times the
query interval (3 minutes). This timeout interval means that the device may continue to forward multicast
traffic onto the subnet for up to 3 minutes after all hosts have left the multicast group.
IGMPv2 Leave Process
IGMPv2 incorporates a leave-group message that provides the means for a host to indicate that it wishes to
stop receiving multicast traffic for a specific group. When an IGMPv2 host leaves a multicast group, if it was
the last host to respond to a query with a membership report for that group, it sends a leave-group message
to the all-devices multicast group (224.0.0.2).
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
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Configuring IGMP
IGMP Join and Leave Process
