Understanding Igmp Version 3 Support; Understanding The Igmp Snooping Querier - Cisco 7604 Configuration Manual

Chapter 30 Configuring IGMP Snooping for IPv4 Multicast Traffic
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 router waits before updating the table entry is called the "last member query
interval." To configure the interval, enter the ip igmp snooping last-member-query-interval interval
command.
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 the IGMP Snooping Querier

Use the IGMP snooping querier 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 where IP multicast routing is configured, the IP multicast router acts as the IGMP querier.
If the IP-multicast traffic in a VLAN only needs to be Layer 2 switched, an IP-multicast router is not
required, but without an IP-multicast router on the VLAN, you must configure another router as the
IGMP querier so that it can send queries.
When enabled, the IGMP snooping querier sends out periodic IGMP queries that trigger IGMP report
messages from the router that wants to receive IP multicast traffic. IGMP snooping listens to these IGMP
reports to establish appropriate forwarding.
Configure one router as the IGMP snooping querier in each VLAN that is supported on switches that use
IGMP to report interest in IP multicast traffic.
Note
You can configure a router to generate IGMP queries on a VLAN regardless of whether or not IP
multicast routing is enabled.

Understanding IGMP Version 3 Support

These sections describe IGMP version 3 support:
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•
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OL-4266-08
Enable the IGMP snooping querier on only one router in the VLAN.
IGMP Version 3 Support Overview, page 30-6
IGMPv3 Fast-Leave Processing, page 30-6
Proxy Reporting, page 30-6
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
Understanding How IGMP Snooping Works
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