Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member



Our campus Network that consist of two Catalyst 6509 on the core became very very very slow. Loging to the console port on one of our Catalyst 6509 display symbol caracteres [looks like Matrix movie].

The two core Switch 6509 [router] where running at 100%of CPU "sh proc cpu" and the high process was "arp input".

The only error we have on the logging is:

%MCAST-2-IGMP_Fallback error.

We identified the Server who was creating this and shutdown the port but the problem was the same, we had to reboot the Catalyst 6509 that slowed the magic caracteres to fix the problem.

My questions are:

1) Is "%MCAST-2-IGMP_Fallback" error can cause the slow down to all Network?

2) Where can I found more doc or a better explanation regarding this error?

3) The Server that was related to this error was "UNIX running IRIX with gated enable", is gated deamon the problem?

4) According to the doc that I am reading, this error is displayed when IGMP snooping is enable. If I disable IGMP snooping what I am going to loose?

Many Thanks



For a detailed explanation of the error message see the "Common error message on Cat6500 switches" document.

Can this cause high CPU ? Yes, if the server is

sending a lot of multicast traffic to a reserved multicast mac address.

You are not saving yourself by disabling IGMP Snooping. IGMP fallback mode is a way for the switch CPU to protect itself from the excess multicast traffic sent to the reserved multicast mac.

Disabling IGMP Snooping will not protect you from a person who sends multicast traffic to a reserved multicast mac address. In fact disabling IGMP Snooping is not good because now all multicast traffic is treated like broadcast traffic, and could result in user port congestion.

CreatePlease login to create content