C
HAPTER
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| Multicast Filtering Commands
IGMP Snooping
– 909 –
◆ If a topology change notification (TCN) is received, and all the uplink
ports are subsequently deleted, a timeout mechanism is used to delete
all of the currently learned multicast channels.
◆ When a new uplink port starts up, the switch sends unsolicited reports
for all current learned channels out through the new uplink port.
◆ By default, the switch immediately enters into “multicast flooding
mode” when a spanning tree topology change occurs. In this mode,
multicast traffic will be flooded to all VLAN ports. If many ports have
subscribed to different multicast groups, flooding may cause excessive
loading on the link between the switch and the end host. Flooding may
be disabled to avoid this, causing multicast traffic to be delivered only
to those ports on which multicast group members have been learned.
◆ When the spanning tree topology changes, the root bridge sends a
proxy query to quickly re-learn the host membership/port relations for
multicast channels. The root bridge also sends an unsolicited Multicast
Router Discover (MRD) request to quickly locate the multicast routers in
this VLAN.
The proxy query and unsolicited MRD request are flooded to all VLAN
ports except for the receiving port when the switch receives such
packets.
EXAMPLE
The following example enables TCN flooding.
Console(config)#ip igmp snooping tcn-flood
Console(config)#
ip igmp snooping
tcn-query-solicit
This command instructs the switch to send out an IGMP general query
solicitation when a spanning tree topology change notification (TCN)
occurs. Use the no form to disable this feature.
SYNTAX
[no] ip igmp snooping tcn-query-solicit
DEFAULT SETTING
Disabled
COMMAND MODE
Global Configuration
COMMAND USAGE
◆ When the root bridge in a spanning tree receives a topology change
notification for a VLAN where IGMP snooping is enabled, it issues a
global IGMP leave message (query solicitation). When a switch receives
this solicitation, it floods it to all ports in the VLAN where the spanning
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