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Uplink going down

At our campus we have strange issue with 3550 switch. This switch has always blocked uplink port (this is connected via trunk) - port is always shutdown - after restart (or when is port manualy set to up) few hours is everything OK - but uplink port is blinkink very fast, but later port is disabled again. Any ideas, some STP issue any ideas how to fix it???? We have this problem with two switches and both are 3550.....


Re: Uplink going down

From your question, it looks as if the port goes into error-disabled state. You a easily verify this by doing a "sh int" for the corresponding port.

This might reveal also the reason why it goes into this state. I can see two possibilties:

1: there is an STP-loop that is broken when this port switches off.

2: You have a media problem on the port. Could be either a duplex mismatch or cabling / grounding.

It would be interesting to know what type of port it concerns (10/100Mb or Gigabit) and it's configuration. Posting a "sh int" of the port in it's disabled state would also be useful. Feel free to post any other info if this does not resolve your question.




Re: Uplink going down

I found in log

%ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on GigabitEthernet0/1. The port is forced to linkdown.

Cisco says to this

These messages are a result of a failure of the switch software when trying to program the hardware.

Most of these errors lead to incorrect switch behavior, and you should call your Cisco technical support representative."

Error Message ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on [chars].

Explanation This message means that a loopback condition might be the

result of a balun cable incorrectly connected into a port. [chars] is the

interface name. Recommended Action Check the cables. If a balun cable is connected and

the loopback condition is desired, no action is required. Otherwise, connect

the correct cable, and then enable the port."

Community Member

Re: Uplink going down


The problem occurs because the keepalive packet is looped back to the port that sent the keepalive. There is a loop in the network. Although disabling the keepalive will prevent the interface from being errdisabled, it will not remove the loop.

Keepalives are sent on the Catalyst 2940, 2950, 2950-LRE, 2955, 2970, 3550, 3560 or 3750 switch to prevent loops in the network. The primary reason for the keepalives is to prevent loops as a result of Type 2 cabling.

Hope this helps.



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