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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 20 Configuring IGMP Snooping and MVR
Configuring IGMP Snooping
If the multicast router query also includes requests for IGMPv3 reports, the switch forwards all IGMPv1,
IGMPv2, and IGMPv3 reports for a group to the multicast devices.
If you disable IGMP report suppression, all IGMP reports are forwarded to the multicast routers.

Source-Only Networks

In a source-only network, switch ports are connected to multicast source ports and multicast router ports.
The switch ports are not connected to hosts that send IGMP join or leave messages.
The switch learns about IP multicast groups from the IP multicast data stream by using the source-only
learning method. The switch forwards traffic only to the multicast router ports.
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.
In addition to IGMP query packets, the switch also uses Protocol-Independent Multicast protocol version
2 (PIMv2) packets for multicast router discovery. The packets are sent to the switch CPU, which can
result in a occasional high CPU traffic. You can disable multicast router discovery by PIMv2 packets by
using the no ip igmp snooping mrouter learn pim v2 global configuration command. This command
only works when you also disable source-only learning on the switch 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 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 20-7
• Enabling or Disabling IGMP Snooping, page 20-7
• Setting the Snooping Method, page 20-8
• Configuring a Multicast Router Port, page 20-9
• Configuring a Host Statically to Join a Group, page 20-9
• Enabling IGMP Immediate-Leave Processing, page 20-10
• Configuring the IGMP Leave Timer, page 20-10
• Disabling IGMP Report Suppression, page 20-11
• Disabling IP Multicast-Source-Only Learning, page 20-12