After the R4001 is configured, the RLOS indicator of the cE1/PR1 interface is still on, indicating disconnection of the physical layer. Replacing with a new cable does not change the symptom, so it is presumed that the cE1/PRI interface is faulty. But the self-loop test on the cE1/PRI interface results that the RLOS indicator is still on.
1) After the physical connection cable is replaced, the RLOS indicator is still on.
2) Replacing with a new router does not change the symptom.
3) Later, it is found that the clock type is not changed during the self-loop test (The cE1/PRI interface with the PPP encapsulation needs the clock for the self-loop).
4) Configure clock dce in interface view, and this problem is solved.
The RLOS indicator being on indicates that the interface does not receive any signal, or receives signals with too many errors. It may not be caused by the bad quality of connection cables. For this kind of problem, first check the physical connection cables for any error. If no problem, check the interface module. If it is normal, check the configuration for errors.