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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter21 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.
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. Y ou ca n di sable mu lti cast rout er disc o v ery b y PIMv 2 pack ets b y
using the no ip igmp snooping mrouter learn pim v2 global configuration command. This command
only works when you also disable source-only l ear nin g on the s witc h b y usi ng th e 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 -onl y le arni ng o r by us ing the IG MP joi n
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 im pact swi tc h p erfor manc e.
If aging is disabled and you want to delete multicast address es tha t t he sw itch le ar ned by u sin g
source-only learning, re-enable aging of the forwarding-ta ble en tri es. T he s witch 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 21-6
•Enabling or Disabling IGMP Snooping, page 21-7
•Setting the Snooping Method, page 21-8
•Configuring a Multicast Router Port, page 21-9
•Configuring a Host Statically to Join a Group, page 21-10
•Enabling IGMP Immediate-Leave Processing, page 21-10
•Disabling IGMP Report Suppression, page 21-11
•Disabling IP Multicast-Source-Only Learning, page 21-11
•Configuring the Aging Time, page 21-12

Default IGMP Snooping Configuration

Table21-3 shows the de fault I GMP snoo ping configur ation .
Table21-3 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.