Serial line troubleshooting

Introduction

This chapter presents general troubleshooting information and a discussion of tools and techniques for troubleshooting serial connections. The chapter consists of the following sections:

• Troubleshooting Using the show interfaces serial Command
  
• Using the show controllers Command
  
• Using debug Commands
  
• Using Extended ping Tests
  
• Troubleshooting Clocking Problems
  
• Adjusting Buffers
  
• Special Serial Line Tests
  
• Detailed Information on the show interfaces serial Command
  
• Troubleshooting T1 Problems
  
• Troubleshooting E1 Problems

Prerequisites

Requirements

Readers of this document should be knowledgeable of the following definitions.

• DTE = data terminal equipment
  
• CD = Carrier Detect
  
• CSU = channel service unit
  
• DSU = digital service unit
  
• SCTE = serial clock transmit external
  
• DCE = data circuit-terminating equipment
  
• CTS = clear-to-send
  
• DSR = data-set ready
  
• SAP = Service Advertising Protocol
  
• IPX = Internetwork Packet Exchange
  
• FDDI = Fiber Distributed Data Interface
  
• ESF = Extended Superframe Format
  
• B8ZS = binary eight-zero substitution
  
• LBO = Line Build Out

Components Used

This document is not restricted to specific software and hardware versions.
The information presented in this document was created from devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If you are working in a live network, ensure that you understand the potential impact of any command before using it.

Conventions

For more information on document conventions, see the Cisco Technical Tips Conventions.

Troubleshooting Using the show interfaces serial Command

The output of the show interfaces serial EXEC command displays information specific to serial interfaces. Figure 15-1 shows the output of the show interfaces serial EXEC command for a High-Level Data Link Control (HDLC) serial interface.
This section describes how to use the show interfaces serial command to diagnose serial line connectivity problems in a wide area network (WAN) environment. The following sections describe some of the important fields of the command output.
Other fields shown in the display are described in detail in the section "Detailed Information on the show interfaces serial Command," later in this chapter.

Serial Lines: show interfaces serial Status Line Conditions

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: Serial Lines: show interfaces serial Status Line Conditions - This table shows the interface status conditions, possible problems associated with the conditions, and solutions to those problems.

Status Line Condition	Possible Problem	Solution
Serial x is up, line protocol is up		This is the proper status line condition. No action required.
Serial x is down, line protocol is down (DTE mode)	Typically indicates that the router is not sensing a CD signal (that is, CD is not active). Telephone company problem-Line is down or line is not connected to CSU/DSU Faulty or incorrect cabling Hardware failure (CSU/DSU)	Check the LEDs on the CSU/DSU to see if CD is active, or insert a breakout box on the line to check for the CD signal. Verify that you are using the proper cable and interface (see your hardware installation documentation). Insert a breakout box and check all control leads. Contact your leased-line or other carrier service to see if there is a problem. Swap faulty parts. 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 (SCTE not set on CSU/DSU) Failed local or remote CSU/DSU Failed local or remote CSU/DSU Router hardware failure (local or remote)	Put the modem, CSU, or DSU in local loopback mode and use the show interfaces serial command to see if the line protocol comes up. If the line protocol comes up, a telephone company problem or a failed remote router is the likely problem. If the problem appears to be on the remote end, repeat Step 1 on the remote modem, CSU, or DSU. Verify all cabling. Make sure 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. Enable the debug serial interface EXEC command. Caution: Because debugging output is assigned a 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 low network traffic and fewer users. Debugging during these periods decreases the likelihood that increased debug command processing overhead will affect system use. 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. 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. 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 (DCE mode)	Missing clockrate interface configuration command DTE device does not support or is not set up for SCTE mode Failed remote CSU or DSU Failed or incorrect cable Router hardware failure	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. Set the DTE device to SCTE mode if possible. If your CSU/DSU does not support SCTE, you may have to disable SCTE on the Cisco router interface. See the section "Inverting the Transmit Clock," later in this chapter. Verify that the correct cable is being used. If the line protocol is still down, there is a possible hardware failure or cabling problem. Insert a breakout box and observe leads. Replace faulty parts as necessary.
Serial x is up, line protocol is up (looped)	A loop exists in the 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.	Use the show running-config privileged EXEC command to look for any loopback interface configuration command entries. If you find a loopback interface configuration command entry, use the no loopback interface configuration