27-3
Software Configuration Guide—Release IOS XE 3 6.0E and IOS 15.2(2)E
OL_28731-01
Chapter 27 Configuring IGMP Snooping and Filtering, and MVR
About IGMP Snooping
For a Layer 2 IGMPv2 host interface to join an IP multicast group, a host sends an IGMP membership
report for the IP multicast group. For a host to leave a multicast group, it can either ignore the periodic
IGMP general queries or it can send an IGMP leave message. When the switch receives an IGMP leave
message from a host, it sends out an IGMP group-specific query to determine whether any devices
connected to that interface are interested in traffic for the specific multicast group. The switch then
updates the table entry for that Layer 2 multicast group so that only those hosts interested in receiving
multicast traffic for the group are listed.
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.
Note By default all IGMP joins are forwarded to all multicast router ports.
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.
Note When using IGMPv2 snooping, use immediate-leave processing only on VLANs where just one host is
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.
Note IGMPv3 is interoperable with older versions of IGMP.
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.
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