You find that one of the serial interfaces on your router is not forwarding traffic. You have given "show ip interfaces" command. In the response, you observed the following entries against that interface:
Serial 0 is up, line protocol is down. What is the likely problem?
- There is a problem at physical layer
- There is a problem exchanging the routing information
- The link was administratively down
- There is a problem with the route both at physical and protocol layers