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11-5
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter 11 Configuring IGMP Snooping and MVR
Configuring IGMP Snooping
Note You should only use the Immediate-Leave processing feature on VLANs where a single host is
connected to each port. If Immediate Leave is enabled in VLANs where more than one host is
connected to a port, some hosts might be inadvertently dropped. Immediate Leave is supported with
only IGMP version 2 hosts.
Configuring IGMP Snooping
IGMP snooping allows switches to examine IGMP packets and make forwarding decisions based on
their content. To enable IGMP snooping on the switch to discover external multicast routers, the Layer 3
interfaces on the routers in the VLAN must already have been configured for multicast routing. For more
information, see Chapter 24, “Configuring IP Multicast Routing.”
These sections describe how to configure IGMP snooping:
• Default IGMP Snooping Configuration, page 11-5
• Enabling or Disabling IGMP Snooping, page 11-5
• Setting the Snooping Method, page 11-6
• Configuring a Multicast Router Port, page 11-7
• Configuring a Host Statically to Join a Group, page 11-8
• Enabling IGMP Immediate-Leave Processing, page 11-9
Default IGMP Snooping Configuration
Table 11-3 shows the default IGMP snooping configuration.
Enabling or Disabling IGMP Snooping
By default, IGMP snooping is globally enabled on the switch. When globally enabled or disabled, it is
also enabled or disabled in all existing VLAN interfaces. IGMP snooping is by default enabled on all
VLANs, but can be enabled and disabled on a per-VLAN basis. After you configure the VLAN interface
for multicast routing, no configuration is needed for the switch to dynamically access external multicast
routers by using IGMP snooping.
Global IGMP snooping overrides the VLAN IGMP snooping. If global snooping is disabled, you cannot
enable VLAN snooping. If global snooping is enabled, you can enable or disable VLAN snooping.
Table 11-3 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