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Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
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Chapter 21 Configuring IGMP Snooping
Understanding How IGMP Snooping Works
response to the general query, it removes the group entry and relays the IGMP leave to the multicast
router. If the multicast router receives no reports from a VLAN, the multicast router removes the group
for the VLAN from its IGMP cache.
The interval for which the switch waits before updating the table entry is called the “last member query
interval.” Enter the ip igmp snooping last-member-query-interval interval command to configure the
interval.
Fast-Leave Processing
IGMP snooping fast-leave processing allows IGMP snooping to remove a Layer 2 LAN interface from
the forwarding-table entry without first sending out IGMP group-specific queries to the interface. Upon
receiving a group-specific IGMPv2 leave message, IGMP snooping immediately removes the interface
from the Layer 2 forwarding table entry for that multicast group, unless a multicast router was learned
on the port. Fast-leave processing improves bandwidth management for all hosts on a switched network.
Note Use fast-leave processing only on VLANs where only one host is connected to each Layer 2 LAN port.
If fast-leave is enabled in VLANs where more than one host is connected to a Layer 2 LAN port, some
hosts might be dropped inadvertently. Fast-leave processing is supported only with IGMP version 2
hosts.
Understanding IGMP Snooping Querier
IGMP snooping querier should be used to support IGMP snooping in a VLAN where PIM and IGMP are
not configured because the multicast traffic does not need to be routed.
In a network with IP multicast routing, the IP multicast router acts as the IGMP querier. If the
IP-multicast traffic in a VLAN needs to be Layer 2 switched only, an IP-multicast router is not required,
but without an IP-multicast router on a VLAN, you must configure another switch as the IGMP querier
so that it can send queries.
When IGMP snooping querier is enabled, the IGMP snooping querier sends out periodic IGMP queries
that trigger IGMP report messages from the switch that wants to receive IP multicast traffic. IGMP
snooping listens to these IGMP reports to establish appropriate forwarding.
You can enable the IGMP snooping querier on all the Cisco 7600 series routers in the VLAN, but for
each VLAN that is connected to switches that use IGMP to report interest in IP multicast traffic, you
must set at least one switch as the IGMP snooping querier.
You can use Cisco IOS commands to configure the Cisco 7600 series routers to generate such IGMP
queries on a VLAN regardless of whether or not IP multicast routing is enabled.
Note To enable IP multicast routing on the Cisco 7600 series routers on a specific VLAN, enter the ip pim
sparse-mode command, the ip pim sparse-dense-mode command, or the ip pim dense-mode command
on that interface. See Chapter 18, “Configuring IP Multicast Layer 3 Switching” for more details.
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