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New Member

Serial Line Protocol Problem

I have Two sites A and B, connected through a lease line with PPP encapsulation. I am having a problem with getting both sides up so i have changed the encapsulation to HDLC and one of the sites A goes UP and Down(Flapping) and site B goes up for a seconds and then goes down and stays down. I have changes the serial cables on both sides and the HWIC card still without any luck. Any suggestion will be appreciated.

Hall of Fame Super Gold

Re: Serial Line Protocol Problem

Most likely your leased line is not working, check show interface.

New Member

Re: Serial Line Protocol Problem

I have contacted ISP and they said that there is no problem in the lease line. Also when i am giving the command

show ip int bri the status is up and the protocol is down but in site A its going up and down with the HDLC encapsulation but with PPP encapsulation the it goes on for few seconds and the goes down and stay down.

Hall of Fame Super Gold

Re: Serial Line Protocol Problem

ISP will always say the there is no problem, but actually there is, as you can see yourself.

Anyway, you need always to include show interface when reporting circuit problems.

Cisco Employee

Re: Serial Line Protocol Problem

Hello Karam,

Changing the encapsulation type will not help you. The short up/down flap when you change the encapsulation type is normal and it follows from the keepalive semantics of a particular Layer2 protocol.

If you are certain that your leased line is OK then please revert both your routers back to the PPP and try to see if any PPP negotiation is taking place by entering the commands

debug ppp negotiation

debug ppp error

debug ppp authentication

debug ppp packet ! only if all other fails to produce useful info

Also check the "show interfaces" counters on your Serial interfaces to see if there are any transmission problems recorded (errors, overruns, drops, CRC errors and so on) - these would indicate a problem with the link or with the DCE device.

And also, using the show controllers serial 0/0, check whether the DCE appears connected and whether the TX and RX clocks are detected (substitute the serial 0/0 with your real interface). I assume that the clocking is provided by the DCE.

Best regards,


New Member

Re: Serial Line Protocol Problem

Hello Peter,

I have changed the encapsulation on both ends to PPP and tried out the debug commands that you listed above. I have attached 3 files can you please take a look at it and let me know your feedback.

Hall of Fame Super Gold

Re: Serial Line Protocol Problem

Likely the other side does not receive anything, please take "show controller E1" and "show interface" on the other side.

Hall of Fame Super Silver

Re: Serial Line Protocol Problem

Hello Karam,

as Paolo has noted it should be a provider issue.

It looks like one side doesn't receive the frames from the remote DTE router.

to check this use

debug serial interface.

call the provider and have them make the loop tests with you.

keep the same encapsulation type on both routers.

Hope to help