LAPB (or X.25) of two sides always being down
Symptom
Link layer protocol LAPB (or X.25) of two sides is always down.
Analysis
A possible reason is the two sides are operating in the same mode (DTE or DCE).
Solution
Enable the debugging on both sides. If one side sends SABM frames and the other sends FRMR frames cyclically, the two sides are operating in the same mode (DTE or DCE). Change the operating mode of one side to solve it.