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Community Member

loop back error

hi ,

A LAN is connected to VLAN configured in sw3560 which usually gives error of loopback and shutting down the port.

when we go through CNA , a notification gives a msg

" check configuration of the switch , also make sure devices connected to the sw are not mirroring the traffic back to switch. Enable this port again.

Fastethernet 0/5 is down, line protocol is down (err-disabled)."

now after enabling the port, it is working fine again. this problem is coming only in single vlan (port) among 6 vlan's.

we have changed cable and even sw also.

so, loopback and mirroring concept is similar or differ, pse clear and also suggest the possible solution.



Cisco Employee

Re: loop back error

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,

Community Member

Re: loop back error

hi . tks for good suggestion..

Is there may be this reason also that is

if a loop back adapter is configured in some of the pc inside the LAN.

this is may become the reason for loopback error.


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