When you are using PPP encapsulation a loopback will not show protocol up, HDLC will. This is normal. The fact that you can't connect your 2 routers back to back with either protocol suggest that you need a cross-over cable. You don't say if you have external DSU/CSUs or T1 WICs, but in either case you could get more useful information doing loopbacks on the live circuit. What kind of errors did you see on your interfaces? How is your clocking set? You need to know who is providing the clocking for the T1. In some cases one of your routers needs to be set for internal clock, and sometimes the carrier is clocking so both routers would be set for line.
My guess would be that you have a problem with your T1, but you still need to prove it. Can you request that the phone company test the T1?
You had said that you started seeing this recently, suggesting it was working. If no changes have been made to the router config, it is most likely a T1 problem, although it is possible that a hardware issue exists.
When you have a point to point T1, it is sometimes called a "dry" T1, meaning the telco provides no clock source. In a "dry" T1 scenario, you should set one of your CSU's to Internal clocking and the other CSU to clock from the line. Other times the T1 provider tells you that he is providing clocking for you, so you would just set both your CSUs to "line". The default setting on most CSUs is "line".
Do you have external or internal CSUs? You say you don't have a telco. That doesn't make sense as you had to have had the T1 provisioned by somebody (usually a telco).
1. Put the config of the router back to the way they were when it last worked.
2. Read you CSU documenation on how to do loopbacks.
Do a data port loopback (also called a local loopback) at both sites. You should see the serial interface of the router go UP/UP. If not, you could have a serial cable problem, CSU issue, or router serial port issue.
3. You can do a hardware loopback at the CSU (see below URL) at both sites. This will test the CSU all the way back to your router serial port. If you see the serial port go up/up at both sites, you have a telco issue.
Usually at this point you'd contact the telco and ask them to try and loop the CSU at each site. If they can't, they will try and loop the smartjack at each site. The smartjack is a box on the wall where the telco termininates the T1. You then connect a cable from the smartjack to your CSU.
This will usually shed more light as to where the problem is.
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