Immediate-Leave Processing - Cisco Catalyst 4500 Series Configuration Manual

Chapter 25 Configuring IGMP Snooping and Filtering
In contrast, IGMPv3 hosts send IGMPv3 membership reports (with the allow group record mode) to join
a specific multicast group. When IGMPv3 hosts send membership reports (with the block group record)
to reject traffic from all sources in the previous source list, the last host on the port is removed by
immediate-leave if EHT is enabled.

Immediate-Leave Processing

IGMP snooping immediate-leave processing allows the switch to remove an interface from the
forwarding-table entry without first sending out IGMP group-specific queries to the interface. The
VLAN interface is pruned from the multicast tree for the multicast group specified in the original IGMP
leave message. Immediate-leave processing ensures optimal bandwidth management for all hosts on a
switched network, even when multiple multicast groups are being used simultaneously.
When a switch with IGMP snooping enabled receives an IGMPv2 or IGMPv3 leave message, it sends
an IGMP group-specific query from the interface where the leave message was received to determine
when other hosts are attached to that interface that are interested in joining the MAC multicast group. If
the switch does not receive an IGMP join message within the query response interval, the interface is
removed from the port list of the (MAC-group, VLAN) entry in the Layer 2 forwarding table.
By default all IGMP joins are forwarded to all multicast router ports.
Note
With immediate-leave processing enabled on the VLAN, an interface can be removed immediately from
the port list of the Layer 2 entry when the IGMP leave message is received, unless a multicast router was
learned on the port.
When using IGMPv2 snooping, use immediate-leave processing only on VLANs where just one host is
Note
connected to each interface. If immediate-leave processing is enabled on VLANs where multiple hosts
are connected to an interface, some hosts might be dropped inadvertently. When using IGMPv3,
immediate-leave processing is enabled by default, and due to explicit host tracking, the switch can detect
when a port has single or multiple hosts maintained by the switch for IGMPv3 hosts. As a result, the
switch can perform immediate-leave processing when it detects a single host behind a given port.
IGMPv3 is interoperable with older versions of IGMP.
Note
To display the IGMP version on a particular VLAN, use the show ip igmp snooping querier vlan
command.
To display whether the switch supports IGMPv3 snooping, use the show ip igmp snooping vlan
command.
To enable immediate-leave for IGMPv2, use the ip igmp snooping immediate-leave command.
Immediate-leave processing is enabled by default for IGMPv3.
Note
OL-25340-01
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
About IGMP Snooping
25-3