Cisco IE-4000 Software Configuration Manual page 434

Configuring IGMP Snooping and MVR
Information About IGMP Snooping and MVR

Snooping on IGMP queries, Protocol Independent Multicast (PIM) packets, and Distance Vector Multicast Routing
Protocol (DVMRP) packets
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Listening to Cisco Group Management Protocol (CGMP) packets from other routers
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Statically connecting to a multicast router port with the ip igmp snooping mrouter global configuration command
You can configure the switch either to snoop on IGMP queries and PIM/DVMRP packets or to listen to CGMP self-join or
proxy-join packets. By default, the switch snoops on PIM/DVMRP packets on all VLANs. To learn of multicast router ports
through only CGMP packets, use the ip igmp snooping vlan vlan-id mrouter learn cgmp global configuration command.
When this command is entered, the router listens to only CGMP self-join and CGMP proxy-join packets and to no other
CGMP packets. To learn of multicast router ports through only PIM-DVMRP packets, use the ip igmp snooping vlan
vlan-id mrouter learn pim-dvmrp global configuration command.
Note: If you want to use CGMP as the learning method and no multicast routers in the VLAN are CGMP proxy-enabled,
you must enter the ip cgmp router-only command to dynamically access the router.
Multicast Flooding Time After a TCN Event
You can control the time that multicast traffic is flooded after a topology change notification (TCN) event by using the ip
igmp snooping tcn flood query count global configuration command. This command configures the number of general
queries for which multicast data traffic is flooded after a TCN event. Some examples of TCN events are when the client
changed its location and the receiver is on same port that was blocked but is now forwarding, and when a port went
down without sending a leave message.
If you set the TCN flood query count to 1 by using the ip igmp snooping tcn flood query count command, the flooding
stops after receiving 1 general query. If you set the count to 7, the flooding continues until 7 general queries are received.
Groups are relearned based on the general queries received during the TCN event.
Flood Mode for TCN
When a topology change occurs, the spanning-tree root sends a special IGMP leave message (also known as global
leave) with the group multicast address 0.0.0.0. However, when you enable the ip igmp snooping tcn query solicit
global configuration command, the switch sends the global leave message whether or not it is the spanning-tree root.
When the router receives this special leave, it immediately sends general queries, which expedite the process of
recovering from the flood mode during the TCN event. Leaves are always sent if the switch is the spanning-tree root
regardless of this configuration command. By default, query solicitation is disabled.
Multicast Flooding During a TCN Event
When the switch receives a TCN, multicast traffic is flooded to all the ports until 2 general queries are received. If the
switch has many ports with attached hosts that are subscribed to different multicast groups, this flooding might exceed
the capacity of the link and cause packet loss. You can use the ip igmp snooping tcn flood interface configuration
command to control this behavior.
IGMP Snooping Querier Guidelines

Configure the VLAN in global configuration mode.
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Configure an IP address on the VLAN interface. When enabled, the IGMP snooping querier uses the IP address as
the query source address.
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If there is no IP address configured on the VLAN interface, the IGMP snooping querier tries to use the configured
global IP address for the IGMP querier. If there is no global IP address specified, the IGMP querier tries to use the
VLAN switch virtual interface (SVI) IP address (if one exists). If there is no SVI IP address, the switch uses the first
available IP address configured on the switch. The first IP address available appears in the output of the show ip
interface privileged EXEC command. The IGMP snooping querier does not generate an IGMP general query if it
cannot find an available IP address on the switch.
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