28-4
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 28 Configuring IGMP Snooping and Filtering, and MVR
About IGMP Snooping
IGMP Configurable-Leave Timer
Immediate-leave processing cannot be used on VLANs where multiple hosts may be connected to a
single interface. To reduce leave latency in such a scenario, IGMPv3 provides a configurable leave timer.
In Cisco IOS Release 12.2(25)SG and earlier, the IGMP snooping leave time was based on query
response time. If membership reports were not received by the switch before the query response time of
the query expired, a port was removed from the multicast group membership.
In Cisco IOS Release 12.2(31)SG and later, you can configure the length of time that the switch waits
after sending a group-specific query to determine if hosts are still interested in a specific multicast group.
The IGMP leave response time can be configured from 100 to 5000 milliseconds. The timer can be set
either globally or per-VLAN. The VLAN configuration of the leave time overrides the global
configuration.
For configuration steps, see the “Configuring the IGMP Leave Timer” section on page 28-9.
IGMP Snooping Querier
IGMP Snooping Querier support was introduced in Cisco IOS Release 12.2(50)SG. This is a Layer 2
feature required to support IGMP snooping in a VLAN where PIM and IGMP are not configured because
the multicast traffic does not require routing.
In a network where IP multicast routing is configured, the IP multicast router acts as the IGMP querier
by sending general queries. If the IP-multicast traffic in a VLAN only needs to be Layer 2-switched, an
IP-multicast router is not required. Without an IP-multicast router on the VLAN, you must configure
another switch as the IGMP querier so that it can send queries.
When enabled, the IGMP snooping querier sends out periodic IGMPv2 queries that trigger IGMP report
messages from the switch that requests IP multicast traffic. IGMP snooping listens to these IGMP reports
to establish appropriate forwarding.
On switches that use IGMP to report interest in IP multicast traffic, configure at least one switch as the
IGMP snooping querier in each supported VLAN.
You can configure a switch to generate IGMP queries on a VLAN regardless of whether IP multicast
routing is enabled.
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. EHT also 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.
To determine whether EHT is enabled on a VLAN, use the show ip igmp snoop vlan command.
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