Branch Router(cisco ISR4321/K9) is currently working on secondary link and the primary interface is having issue. After taking console of the router we found there was logs getting generated continuously saying serial 1/0/1 is down and serial 1/0/1 is up. we tried doing shut no shut, also rebooted the router but no luck then we removed the serial interface then also the logs did not stop that was strange.
Cisco IOS Software Version 15.5(3)S4b, RELEASE SOFTWARE (fc1)
I saw dcd down on your show this maybe indicate line or modem problem.
Check this other theread maybe will usefull.