Interface Down

[jonk34567]

" Serial 1/0 is up line protocol Down " what is the possible reason for this error ?

A. Ensure that the serial cable is correctly plugged in to the interfaces.

B. Apply the clock rate 56000 configuration command to the serial0/1 interface of R1.

C. Configure the serial0/1 interfaces on R1 and R2 with the no shutdown command.

D. Change the address of the serial0/1 interface of R1 to 192.1.1.4.

E. Change the subnet masks of both interfaces to 255.255.255.240

[Julio E. Moisa]

Hi

Answer: B

Nowadays the interfaces have clockrate configured by default but a time ago the DCE interface should have clockrate configured. 

[jonk34567]

what about " A " ? can it be a answer ? i found this from a website.it says answer is " A " ? 

[Julio E. Moisa]

Hi

Below you will find many possible causes:

Serial x is up, line protocol is down (DTE mode)	A local or remote router is misconfigured. Keepalives are not being sent by the remote router. A leased-line or other carrier service problem has occurred (noisy line or misconfigured or failed switch). A timing problem has occurred on the cable (SCTE5 not set on CSU/DSU). A local or remote CSU/DSU has failed. Router hardware (local or remote) has failed.	1. Put the modem, CSU, or DSU in local loopback mode and use the show interfaces serial command to determine whether the line protocol comes up. If the line protocol comes up, a telephone company problem or a failed remote router is the likely problem. 2. If the problem appears to be on the remote end, repeat Step 1 on the remote modem, CSU, or DSU. 3. Verify all cabling. Make certain that the cable is attached to the correct interface, the correct CSU/DSU, and the correct telephone company network termination point. Use the show controllers exec command to determine which cable is attached to which interface. 4. Enable the debug serial interface exec command.mmm ,,,,
Serial x is up, line protocol is down (DTE mode) (continued)		Caution: Because debugging output is assigned high priority in the CPU process, it can render the system unusable. For this reason, use debug commands only to troubleshoot specific problems or during troubleshooting sessions with Cisco technical support staff. Moreover, it is best to use debug commands during periods of lower network traffic and fewer users. Debugging during these periods decreases the likelihood that increased debug command processing overhead will affect system use. 5. If the line protocol does not come up in local loopback mode, and if the output of the debug serial interface exec command shows that the keepalive counter is not incrementing, a router hardware problem is likely. Swap router interface hardware. 6. If the line protocol comes up and the keepalive counter increments, the problem is not in the local router. Troubleshoot the serial line, as described in the sections "Troubleshooting Clocking Problems" and "CSU and DSU Loopback Tests," later in this chapter. 7. If you suspect faulty router hardware, change the serial line to an unused port. If the connection comes up, the previously connected interface has a problem.
Serial x is up, line protocol is down (DCE6 mode)	The clockrate interface configuration command is missing. The DTE device does not support or is not set up for SCTE mode (terminal timing). The remote CSU or DSU has failed.	1. Add the clockrate interface configuration command on the serial interface. Syntax: clock rate bps Syntax Description: •bps—Desired clock rate in bits per second: 1200, 2400, 4800, 9600, 19200, 38400, 56000, 64000, 72000, 125000, 148000, 250000, 500000, 800000, 1000000, 1300000, 2000000, 4000000, or 8000000.
Serial x is up, line protocol is down (DCE mode) (continued)	The clockrate interface configuration command is missing. The DTE device does not support or is not set up for SCTE mode (terminal timing). The remote CSU or DSU has failed.	2. Set the DTE device to SCTE modem if possible. If your CSU/DSU does not support SCTE, you might have to disable SCTE on the Cisco router interface. Refer to the section "Inverting the Transmit Clock," later in this chapter. 3. Verify that the correct cable is being used. 4. If the line protocol is still down, there is a possible hardware failure or cabling problem. Insert a breakout box and observe leads. 5. Replace faulty parts, as necessary.

https://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1915.html#wp1020563

[therealplacap]

Serial 1/0 is up line protocol Down

Serial 1/0 is up - this part refers to physical layer or layer 1

line protocol Down - the latter refers to data link layer or layer 2

So from the choices,

A. Ensure that the serial cable is correctly plugged in to the interfaces. - Physical layer troubleshooting

B. Apply the clock rate 56000 configuration command to the serial0/1 interface of R1. - Only choice that refers back to layer 2.

C. Configure the serial0/1 interfaces on R1 and R2 with the no shutdown command. - None of the interfaces are "administratively shut"

D. Change the address of the serial0/1 interface of R1 to 192.1.1.4. - Layer 3 troubleshooting

E. Change the subnet masks of both interfaces to 255.255.255.240 - Layer 3 troubleshooting