Hello Mitra,

Ports on Cisco Catalyst switches send periodic frames that uniquely identify the device and the port they are sent from. These frames are called loop detection frames. If a port receives a loop detection frame it has sent out itself then the frame must have indeed returned to its origin port via a loop. In such a case, the port will be err-disabled.

I suppose that this is what is happening to you - a loop detection frame is received by a port that has originated it. The question is, why does this happen. One of possible causes is indeed a loop in your network created by multiple connections between your switches and/or hubs. Another possibility is that you have a device connected on your network that for some inexplicable reason sends out the traffic that it just received (this is what can be called a mirroring) - I can imagine a Linux or Windows machine with multiple NICs in bridging/teaming mode doing this, for example. And yet another option is a forgotten SPAN session running on your switches that returns the captured traffic back to the network for whatever reasons.

I am sorry I cannot help you more precisely but there are frankly many possible ways for this error to happen. The primary principle stands, though - a port receives back its own loop detection frame. The detective work is now necessary why the frame indeed loops back.

Best regards,
Peter