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. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

Serial Interface keeps going up and down

I have a serial interface that goes up and then back down about every 30 seconds. Does anyone know of any debug commands that I can put on this sub interface to determine what could be causing this. I have 14 other remote locations workling fine. I am positive all of the cabling is good. Just kind of at a loss. I looked at the interface and there are packets barely going in and out and they all have errors. thanks in advance for any help!!!!

By the way, it is a 1751


Re: Serial Interface keeps going up and down

The high error rate is not letting keepalives through so the circuit is probably never really coming up. It tries to periodically and those attempts are logged as up / down cycles. Look in the sh int for a high and incrementing interface reset count. That means the router is reseting the circuit because of the keep alive failures. In my experience its usually a carrier problem.

New Member

Re: Serial Interface keeps going up and down


you can write this command on your serial interface .and solve your problem.


but you should check other side router , if in the other side is non-cisco router set your encapsulation on the both serial interface to PPP and if your interface is a transmit only , you should set IGNORE-DCD on the interface that only received .

Hope to do help.




Re: Serial Interface keeps going up and down

I would not recommend doing the ignore dcd or tunring off keep alives , you will neverf know if you have a problem with these commands , it is putting a band aid on the problem instead of correcting the root cause that is most likely a problem with your carrier .

Can you paste in a sh interface for this interface