ATMARP (loopback) Frames being broadcast – Cisco Catalyst 3750 12.2-40 (advipservicesk9)   Leave a comment

I’ve noticed one of my switches is broadcasting ATMARP frames every 10 seconds to all switchports.. I had no idea why, as the switch has no ATM interfaces (naturally) nor does the IOS support ATM/LANE. CDP and STP was disabled.

I think these frames are being mis-decoded in Network Monitor – they also showed as loopback frames in the trace (even though classified as ATMARP), which led me to this page http://www.ams-ix.net/technical/config_guide/config_guide.htm#commonly_seen_illegal_traffic_and_setup

Aha! These frames are sent by default every 10 secs on each ethernet interface as a soft connectivity testing mechanism.

This is an example of someone seeing this kind of traffic – http://www.velocityreviews.com/forums/t38199-damn-loop.html

To disable an interface’s soft loopback diagnostic frame capability, configure "no keepalives" on the interface.

The downside to configuring this is the interface will always show UP/UP when any media is connected – speed/duplex issues will not trigger UP/DOWN.

Wonderful mysteries and magics of IT :)

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Posted May 28, 2008 by benryanau in Cisco

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