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Cisco IE 2000 Switch Software Configuration Guide
OL-25866-01
Chapter 28 Configuring IGMP Snooping and MVR
Information About IGMP Snooping and MVR
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 vl an-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 mult icast 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 sendin g 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 quer y. 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 IG MP 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 l ink 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.
• Configure an IP address on the VLAN interface. When enabled, the IGMP snooping querier uses the
IP address as the query source address.
• 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