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16-6
Catalyst 2940 Switch Software Configuration Guide
78-15507-02
Chapter 16 Configuring IGMP Snooping and MVR
Configuring IGMP Snooping
The default learning method is IP multicast-source-only learning. You can disable IP
multicast-source-only learning by using the no ip igmp snooping source-only-learning global
configuration command.
By default, the switch ages out forwarding-table entries that were learned by the source-only learning
method and that are not in use. If the aging time is too long or is disabled, the forwarding table is filled
with unused entries that the switch learned by using source-only learning or by using the IGMP join
messages. When the switch receives traffic for new IP multicast groups, it floods the packet to all ports
in the same VLAN. This unnecessary flooding can impact switch performance.
If aging is disabled and you want to delete multicast addresses that the switch learned by using
source-only learning, re-enable aging of the forwarding-table entries. The switch can now age out the
multicast addresses that were learned by the source-only learning method and that are not in use.
Configuring IGMP Snooping
IGMP snooping allows switches to examine IGMP packets and make forwarding decisions based on their
content.
These sections describe how to configure IGMP snooping:
• Default IGMP Snooping Configuration, page 16-6
• Enabling or Disabling IGMP Snooping, page 16-7
• Setting the Snooping Method, page 16-7
• Configuring a Multicast Router Port, page 16-8
• Configuring a Host Statically to Join a Group, page 16-9
• Enabling IGMP Immediate-Leave Processing, page 16-10
• Disabling IP Multicast-Source-Only Learning, page 16-11
• Configuring the Aging Time, page 16-12
Default IGMP Snooping Configuration
Table 16-3 shows the default IGMP snooping configuration.
Table 16-3 Default IGMP Snooping Configuration
Feature Default Setting
IGMP snooping Enabled globally and per VLAN.
Multicast routers None configured.
Multicast router learning (snooping) method PIM-DVMRP.
IGMP snooping Immediate Leave Disabled.
Static groups None configured.
IP multicast-source-only learning Enabled.
Aging forward-table entries (when source-only
learning is enabled)
Enabled. The default is 600 seconds (10
minutes).
IGMP report suppression Enabled.