Immediate-Leave Processing; Explicit Host Tracking - Cisco 4500M Software Manual

Chapter 18 Configuring IGMP Snooping and Filtering

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 there are other hosts 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.
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 (see below), 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
Use the show ip igmp snooping querier vlan command to display the IGMP version on a particular
VLAN.
Use the show ip igmp snooping vlan command to display whether or not the switch supports IGMPv3
snooping.
Use the ip igmp snooping immediate-leave command to enable immediate-leave for IGMPv2.
Immediate-leave processing is enabled by default for IGMPv3.
Note

Explicit Host Tracking

Explicit Host Tracking (EHT) monitors group membership by tracking hosts that are sending IGMPv3
membership reports. This tracking enables a switch to detect host information associated with the groups
of each port. Furthermore, EHT enables the user to track the membership and various statistics.
EHT enables a switch to track membership on a per-port basis. Consequently, a switch is aware of the
hosts residing on each port and can perform immediate-leave processing when there is only one host
behind a port.
OL-6696-01
Software Configuration Guide—Release 12.2(25)EW
Overview of IGMP Snooping
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