Serial Lines show interfaces serial Status Line Conditions

Last Updated on Tue, 04 Apr 2023 | Exec Command

You can identify five possible problem states in the interface status line of the show interfaces serial display (see Figure 15-1):

• Serial x is down, line protocol is down

• Serial x is up, line protocol is down

• Serial x is up, line protocol is up (looped)

• Serial x is up, line protocol is down (disabled)

• Serial x is administratively down, line protocol is down

Figure 15-1: Output of the HDLC show interface serial Command

Table 15-1 shows the interface status conditions, possible problems associated with the conditions, and solutions to those problems.

Table 15-1: Serial Lines: show interfaces serial Status Line Conditions

Readers' Questions

ilberic
How to put serial status into up state?

1 year ago
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To put a serial status back into the up state, you will need to reset the device or its internal settings. Depending on the type of device, this could mean performing a factory reset, rebooting the device, powering it off and on again, or adjusting its internal settings. If the device is using a protocol such as Serial over Ethernet (SoE), then you may also need to adjust the settings of the serial connection to return the status to up.

Status Line Condition	Possible Problem	Solution
Serial a is up,	___	This is the proper status line
line protocol		condition. No action required.
is up
Serial a is down, line protocol is down (DTE1 mode)	Typically indicates that the router is not sensing a CD2 signal (that is, CD is not active). • Telephone company problem—Line is down or line is not connected to CSU3/DSU4 • Faulty or incorrect cabling • Hardware failure (CSU/DSU)	Step 1 Check the LEDs on the CSU/DSU to see whether CD is active, or insert a breakout box on the line to check for the CD signal. Step 2 Verify that you are using the proper cable and interface (see your hard- ware installation documentation). Step 3 Insert a breakout box and check all control leads. Step 4 Contact your leased-line or other carrier service to see whether there is a problem. Step 5 Swap faulty parts. Step 6 If you suspect faulty router hardware, change the serial line to another port. If the connection

comes up, the previously connected interface has a problem.

Serial x is up, line protocol is down (DTE mode)

• Local or remote router is misconfigured

• Keepalives are not being sent by remote router

• Leased-line or other carrier service problem---noisy line, or misconfigured or failed switch

• Timing problem on cable (SCTE5 not set on CSU/DSU)

• Failed local or remote CSU/DSU

• Router hardware failure (local or remote)

Step 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.

Step 2 If the problem appears to be on the remote end, repeat Step 1 on the remote modem, CSU, or DSU.

Step 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.

Step 4 Enable the debug serial interface exec command.

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.

Serial x is up, line protocol is down (DTE mode)

Step 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.

Step 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.

Step 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)

• Missing clockrate interface configuration command

• DTE device does not support or is not set up for SCTE mode (terminal timing)

• Failed remote CSU or DSU

• Failed or incorrect cable

• Router hardware failure

Step 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.

Step 2 Set the DTE device to SCTE mode 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.

Step 3 Verify that the correct cable is being used.

Step 4 If the line protocol is still down, there is a possible hardware failure or cabling problem. Insert a breakout box and observe leads.

Step 5 Replace faulty parts as necessary.

Serial x is up, line protocol is up (looped)

Loop exists in circuit. The sequence number in the keepalive packet changes to a random number when a loop is initially detected. If the same random number is returned over the link, a loop exists.

Step 1 Use the show running-config privileged exec command to look for any loopback interface configuration command entries.

Step 2 If you find a loopback interface configuration command entry, use the no loopback interface configuration command to remove the loop.

Step 3 If you do not find the loopback interface configuration command, examine the CSU/DSU to determine whether they are configured in manual loopback mode. If they are, disable manual loopback.

Step 4 Reset the CSU or DSU and inspect the line status. If the line protocol comes up, no other action is needed.

Step 5 If the CSU or DSU is not configured in manual loopback mode, contact the leased-line or other carrier service for line troubleshooting assistance.

Serial x is up, line protocol is down

(disabled)

• High error rate due to telephone company service problem

• CSU or DSU hardware problem

• Bad router hardware (interface)

Step 1 Troubleshoot the line with a serial analyzer and breakout box. Look for toggling CTS7 and DSR8 signals.

Step 2 Loop CSU/DSU (DTE loop). If the problem continues, it is likely that there is a hardware problem. If the problem does not continue, it is likely that there is a telephone company problem.

Step 3 Swap out bad hardware as required (CSU, DSU, switch, local or remote router).

Serial x is administratively down, line protocol is down

• Router configuration includes the shutdown interface configuration command

• Duplicate IP address

Step 1 Check the router configuration for the shutdown command.

Step 2 Use the no shutdown interface configuration command to remove the shutdown command.

Step 3 Verify that there are no identical IP addresses using the show running-config privileged exec command or the show interfaces exec command.

Step 4 If there are duplicate addresses, resolve the conflict by changing one of the IP addresses.

1DTE = data terminal equipment

2CD = Carrier Detect

3CSU = channel service unit

4DSU = digital service unit