Suppressing Multicast Flooding - Cisco Catalyst 4500 series Administration Manual

Configuring IGMP Snooping
This example shows how to configure a host statically in VLAN 200 on interface Fast Ethernet 2/11:
Switch# configure terminal
Switch(config)# ip igmp snooping vlan 200 static 0100.5e02.0203 interface fastethernet
2/11
Configuring port FastEthernet2/11 on group 0100.5e02.0203 vlan 200
Switch(config)# end

Suppressing Multicast Flooding

An IGMP snooping-enabled switch floods 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, the following interface and global commands were
introduced in Cisco IOS Release 12.1(11b)EW:
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Prior to Cisco IOS 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 Cisco IOS Release
12.1(11b)EW, the default time period the switch waits before multicast flooding stops 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 enter 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.
Software Configuration Guide—Release IOS XE 3 6.0E and IOS 15.2(2)E
27-12
[no | default] ip igmp snooping tcn flood (interface command)
[no | default] ip igmp snooping tcn flood query count [1 - 10] (global command)
[no | default] ip igmp snooping tcn query solicit (global command)
Chapter 27 Configuring IGMP Snooping and Filtering, and MVR
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