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17-9
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 17 Configuring IGMP Snooping and Filtering
Configuring IGMP Snooping
Suppressing Multicast Flooding
An IGMP snooping-enabled switch will flood multicast traffic to all ports in a VLAN when a
spanning-tree Topology Change Notification (TCN) is received. Multicast flooding suppression enables
a switch to stop sending such traffic. To support flooding suppression, a new interface command and two
new global commands are introduced in release 12.1(11b)EW.
The new interface command is as follows:
[no | default] ip igmp snooping tcn flood
These are the new global commands:
[no | default] ip igmp snooping tcn flood query count [1 - 10]
[no | default] ip igmp snooping tcn query solicit
Prior to release 12.1(11b)EW, when a spanning tree topology change notification (TCN) was received
by a switch, the multicast traffic was flooded to all the ports in a VLAN for a period of three IGMP query
intervals. This was necessary for redundant configurations. In release 12.1(11b)EW, the default time
period the switch will wait before multicast flooding will stop was changed to two IGMP query intervals.
This flooding behavior is undesirable if the switch that does the flooding has many ports that are
subscribed to different groups. The traffic could exceed the capacity of the link between the switch and
the end host, resulting in packet loss.
With the no ip igmp snooping tcn flood command, you can disable multicast flooding on a switch
interface following a topology change. Only the multicast groups that have been joined by a port are sent
to that port, even during a topology change.
With the ip igmp snooping tcn flood query count command, you can enable multicast flooding on a
switch interface for a short period of time following a topology change by configuring an IGMP query
threshold.
Typically, if a topology change occurs, the spanning tree root switch issues a global IGMP leave message
(referred to as a “query solicitation”) with the group multicast address 0.0.0.0. When a switch receives
this solicitation, it floods this solicitation on all ports in the VLAN where the spanning tree change
occurred. When the upstream router receives this solicitation, it immediately issues an IGMP general
query.
With the ip igmp snooping tcn query solicit command, you can now direct a non-spanning tree root
switch to issue the same query solicitation.
The following sections provide additional details on the new commands and illustrate how you can use
them.
IGMP Snooping Interface Configuration
A topology change in a VLAN may invalidate previously learned IGMP snooping information. A host
that was on one port before the topology change may move to another port after the topology change.
When the topology changes, the Catalyst 4500 series switch takes special actions to ensure that multicast
traffic is delivered to all multicast receivers in that VLAN.
When the spanning tree protocol is running in a VLAN, a spanning tree topology change notification
(TCN) is issued by the root switch in the VLAN. A Catalyst 4500 series switch that receives a TCN in
a VLAN for which IGMP snooping has been enabled immediately enters into “multicast flooding mode”
for a period of time until the topology restabilizes and the new locations of all multicast receivers are
learned.