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. And see here for current known issues.

New Member

Problem bringing up a T1 line

I am certain this is a very basic question but I don't know the answer. I have 2 2811's connected via a private T1 (no Telco in the middle). On one end I am getting an up/down situation on the other down/down. With the exception of setting up the clock to be internal on one end and line on the other I kept the defaults (B8ZS,ESF etc). Secondly on the end with the up/down it does go up/up for maybe 20 seconds and then up/down after that when the interface is brought on line. What exactly is up/down telling me and are there any guesses why it comes up for a few seconds and then goes down?

Thanks in advance.


Re: Problem bringing up a T1 line

Which encaps are you using (Frame, HDLC, or PPP)?

If you are going to/from a WIC, make sure your cable is a T1 crossover (pins 1&2 go to 4&5 respectively)

If you are using serial ports and a "back-to-back" cable, make sure the DCE end of the cable goes to the DCE device (the one generating the clock). The cable should have labels on both ends.

It's frequently helpful if you could post a sanitized version of your "SHOW RUN"

Good Luck, Happy Holidays!


New Member

Re: Problem bringing up a T1 line

Thanks for the response. I am using HDLC. I do not have the config handy and won't until after the holiday but as I mentioned it is largely unconfigured at the moment using all of the defaults. I have been able to test the link with a T-Berd and all is OK. I do have CD on each end. I am using WIC's and I do have the cross-over pinout.

Am I correct in assuming that the protocol down state is referring to HDLC in my case?



Re: Problem bringing up a T1 line


I believe it indicates that the 'keepalive' has not been seen for a while (if ever).

(did you "no shut" both interfaces?)

Good Luck



Re: Problem bringing up a T1 line

Make sure that your timeslots are mapped correctly for both sides. If using all 24 DS0's and if Nx64 or not. If the line comes up shortly, also check the clocking on the line. If no telco then one side needs to clock and the other needs to run external clock. The up/down means that you are not seeing the HDLC keepalive from the remote end. I am sure you have all your DCE signals on your Show Interface Serial commands... Try checking the above..Good Luck...

Re: Problem bringing up a T1 line

Serial 1/0 is UP, line protocol is down

in the above statement, 1st half statement mentions the Physical layer connectivity. If this is UP, then Physical connectivity is ok. else chk cables, modems, interfaces etc.

2nd half is all about Data link layer. Here u need to match encapsulations, keepalives, clockrates & BW on each end.

CreatePlease login to create content