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The "%MCAST-2-IGMP_SNOOP_DISABLE" error message is received in Catalyst 6000 / 6500 switch running CatOS software

Core issue

The error message similar to this appears in the logs:

%MCAST-2-IGMP_SNOOP_DISABLE:IGMP Snooping disabled due to excessive events/packets, 0/19820; auto reenable in about 2 mins

The error message indicates that you receive too many IGMP packets that can potentially cause high CPU and eventually results in denial of service. IGMP snooping basically looks at these control packets that are exchanged between routers and hosts and based on the joins, leaves and queries update what ports receive the multicast.

The multicast application can generate a lot of mcast traffic to the switch that made it disable IGMP snooping, which causes forced packets to the CPU, and in turn, prevented BPDUs to get sent out or received.


As a workaround, you can temporarily disable IGMP snooping for two minutes and then re-enable it to see if the issue has gone away. In order to enable or disable IGMP snooping on the switch, use the set igmp [enable | disable] command.

Complete these steps in order to troubleshoot this problem:

  1. Setup a SPAN session on the management VLAN interface SC0 on your Catalyst 6500 in order to determine that the MAC address belongs to the source from where the excessive traffic is coming from.

  2. Look into the CAM table in order to identify the source, and remove that source.

  3. Remove the indentified source and re-enable IGMP snooping.


Refer to the SPAN on the Catalyst 6500/6000 Series Switches that Run CatOS section of Configuring the Catalyst Switched Port Analyzer (SPAN) Feature for more information on how to configure SPAN.

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Last update:
‎06-22-2009 04:37 PM
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