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20-6
Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 20 Configuring IGMP Snooping and MVR
Configuring IGMP Snooping
The default learning method for traffic that aliases with reserved, destination, multicast IP addresses is
IP multicast-source-only learning. Traffic that does not alias with these multicast addresses is forwarded
to both the multicast source ports and multicast router ports. You cannot disable IP multicast-source-only
learning for the traffic with reserved, destination, multicast IP addresses.
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. 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 34, “Configuring IP Multicast Routing.”
These sections describe how to configure IGMP snooping:
•
Default IGMP Snooping Configuration, page 20-6
•
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
•
Disabling IGMP Report Suppression, page 20-11
•
Configuring the Aging Time, page 20-12
Default IGMP Snooping Configuration
shows the default IGMP snooping configuration.
Table 20-3 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.