Table Of Contents
Troubleshooting Serial Lines
Troubleshooting Using the show interfaces serial Command
Serial Lines: show interfaces serial Status Line Conditions
Serial Lines: Increasing Output Drops on Serial Link
Serial Lines: Increasing Input Drops on Serial Link
Serial Lines: Increasing Input Errors in Excess of 1 Percent of Total Interface Traffic
Serial Lines: Troubleshooting Serial Line Input Errors
Serial Lines: Increasing Interface Resets on Serial Link
Serial Lines: Increasing Carrier Transitions Count on Serial Link
Using the show controllers Command
Using debug Commands
Using Extended ping Tests
Troubleshooting Clocking Problems
Clocking Overview
Clocking Problem Causes
Detecting Clocking Problems
Isolating Clocking Problems
Clocking Problem Solutions
Inverting the Transmit Clock
Adjusting Buffers
Tuning System Buffers
Implementing Hold Queue Limits
Using Priority Queuing to Reduce Bottlenecks
Special Serial Line Tests
CSU and DSU Loopback Tests
CSU and DSU Local Loopback Tests for HDLC or PPP Links
CSU and DSU Remote Loopback Tests for HDLC or PPP Links
Detailed Information on the show interfaces serial Command
show interfaces serial
Syntax Description
Command Mode
Usage Guidelines
Sample Displays
Troubleshooting T1 Problems
Troubleshooting Using the show controller t1 Command
show controller t1 Conditions
Is the Controller Administratively Down?
Is the Line Up?
If Receiver Has Loss of Frame
If Receiver Has Loss of Signal
If the Line Is in Loopback Mode
If the Controller Displays Any Alarms
Receive (RX) Alarm Indication Signal (AIS) (Blue)
Receive (Rx) Remote Alarm Indication (Yellow)
Transmitter Sending Remote Alarm (Red)
Transmit (Tx) Remote Alarm Indication (Yellow)
Transmit (Tx) AIS (Blue)
Troubleshooting Error Events
Slip Secs Counter Is Increasing
Framing Loss Seconds Counter Is Increasing
Line Code Violations Are Increasing
Verify that isdn switchtype and pri-group Are Configured Correctly
Verifying the Signaling Channel
Troubleshooting a PRI
Troubleshooting Using the show isdn status Command
Using debug q921
Performing Hardware Loopback Plug Test
Performing the Loopback Plug Test
Troubleshooting E1 Problems
Troubleshooting Using the show controller e1 Command
Show controller e1 Conditions
Troubleshooting E1 Error Events
Verifying That isdn switchtype and pri-group Are Configured Correctly
Verifying the Signaling Channel
Troubleshooting a PRI
Troubleshooting Using the show isdn status Command
Using debug q921
Troubleshooting Serial Lines
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
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 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
Status Line
Condition
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Possible Problem
|
Solution
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Serial x is up, line protocol is up
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—
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This is the proper status line condition. No action is required.
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Serial x is down, line protocol is down (DTE1 mode)
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The router is not sensing a CD2 signal (that is, the CD is not active).
A telephone company problem has occurred—line is down or is not connected to CSU3 /DSU4 .
Cabling is faulty or incorrect.
Hardware failure has occurred (CSU/DSU).
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1. Check the LEDs on the CSU/DSU to see whether the CD is active, or insert a breakout box on the line to check for the CD signal.
2. Verify that you are using the proper cable and interface (see your hardware installation documentation).
3. Insert a breakout box and check all control leads.
4. Contact your leased-line or other carrier service to see whether there is a problem.
5. Swap faulty parts.
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.
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Serial x is up, line protocol is down (DTE mode)
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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.
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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 ,,,,
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Serial x is up, line protocol is down (DTE mode) (continued)
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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.
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Serial x is up, line protocol is down (DCE6 mode)
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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.
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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.
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Serial x is up, line protocol is down (DCE mode) (continued)
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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.
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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.
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Serial x is up, line protocol is up (looped)
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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.
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1. Use the show running-config privileged exec command to look for any loopback interface configuration command entries.
2. If you find a loopback interface configuration command entry, use the no loopback interface configuration command to remove the loop.
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.
4. Reset the CSU or DSU, and inspect the line status. If the line protocol comes up, no other action is needed.
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.
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Serial x is up, line protocol is down (disabled)
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A high error rate has occurred due to a telephone company service problem.
A CSU or DSU hardware problem has occurred.
Router hardware (interface) is bad.
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1. Troubleshoot the line with a serial analyzer and breakout box. Look for toggling CTS7 and DSR8 signals.
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.
3. Swap out bad hardware, as required (CSU, DSU, switch, local or remote router).
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Serial x is administrat-ively down, line protocol is down
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The router configuration includes the shutdown interface configuration command.
A duplicate IP address exists.
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1. Check the router configuration for the shutdown command.
2. Use the no shutdown interface configuration command to remove the shutdown command.
3. Verify that there are no identical IP addresses using the show running-config privileged exec command or the show interfaces exec command.
4. If there are duplicate addresses, resolve the conflict by changing one of the IP addresses.
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Serial Lines: Increasing Output Drops on Serial Link
Output drops appear in the output of the show interfaces serial command (refer to Figure 15-1) when the system is attempting to hand off a packet to a transmit buffer but no buffers are available.
Symptom: Increasing output drops on serial link
Table 15-2 outlines the possible problem that might cause this symptom and describes solutions to that problem.
Table 15-2 Serial Lines: Increasing Output Drops on Serial Link
Possible Problem
|
Solution
|
Input rate to serial interface exceeds bandwidth available on serial link
|
1. Minimize periodic broadcast traffic, such as routing and SAP1 updates, by using access lists or by other means. For example, to increase the delay between SAP updates, use the ipx sap-interval interface configuration command.
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Input rate to serial interface exceeds bandwidth available on serial link (continued)
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2. Increase the output hold queue size in small increments (for instance, 25 percent), using the hold-queue out interface configuration command.
3. On affected interfaces, turn off fast switching for heavily used protocols. For example, to turn off IP fast switching, enter the no ip route-cache interface configuration command. For the command syntax for other protocols, consult the Cisco IOS configuration guides and command references.
4. Implement priority queuing on slower serial links by configuring priority lists. For information on configuring priority lists, see the Cisco IOS configuration guides and command references.
Note: Output drops are acceptable under certain conditions. For instance, if a link is known to be overused (with no way to remedy the situation), it is often considered preferable to drop packets than to hold them. This is true for protocols that support flow control and can retransmit data (such as TCP/IP and Novell IPX2 ). However, some protocols, such as DECnet and local-area transport, are sensitive to dropped packets and accommodate retransmission poorly, if at all.
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Serial Lines: Increasing Input Drops on Serial Link
Input drops appear in the output of the show interfaces serial exec command (refer to Figure 15-1) when too many packets from that interface are still being processed in the system.
Symptom: Increasing number of input drops on serial link
Table 15-3 outlines the possible problem that might cause this symptom and describes solutions to that problem.
Table 15-3 Serial Lines: Increasing Input Drops on Serial Link
Possible Problem
|
Solution
|
Input rate exceeds the capacity of the router, or input queues exceed the size of output queues
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Note: Input drop problems are typically seen when traffic is being routed between faster interfaces (such as Ethernet, Token Ring, and FDDI1 ) and serial interfaces. When traffic is light, there is no problem. As traffic rates increase, backups start occurring. Routers drop packets during these congested periods.
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Input rate exceeds the capacity of the router, or input queues exceed the size of output queues (continued)
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1. Increase the output queue size on common destination interfaces for the interface that is dropping packets. Use the hold-queue number out interface configuration command. Increase these queues by small increments (for instance, 25 percent) until you no longer see drops in the show interfaces output. The default output hold queue limit is 100 packets.
2. Reduce the input queue size, using the hold-queue number in interface configuration command, to force input drops to become output drops. Output drops have less impact on the performance of the router than do input drops. The default input hold queue is 75 packets.
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Serial Lines: Increasing Input Errors in Excess of 1 Percent of Total Interface Traffic
If input errors appear in the show interfaces serial output (refer to Figure 15-1), there are several possible sources of those errors. The most likely sources are summarized in Table 15-4.
Note Any input error value for cyclic redundancy check (CRC) errors, framing errors, or aborts above 1 percent of the total interface traffic suggests some kind of link problem that should be isolated and repaired.
Symptom: Increasing number of input errors in excess of 1 percent of total interface traffic
Table 15-4 Serial Lines: Increasing Input Errors in Excess of 1 Percent of Total Interface Traffic
Possible Problem
|
Solution
|
The following problems can result in this symptom:
•Faulty telephone company equipment
•Noisy serial line
•Incorrect clocking configuration (SCTE not set)
|
Note: Cisco strongly recommends against the use of data converters when you are connecting a router to a WAN or a serial network.
1. Use a serial analyzer to isolate the source of the input errors. If you detect errors, there likely is a hardware problem or a clock mismatch in a device that is external to the router.
|
•Incorrect cable or cable that is too long
•Bad cable or connection
•Bad CSU or DSU
•Bad router hardware
•Data converter or other device being used between router and DSU
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2. Use the loopback and ping tests to isolate the specific problem source. For more information, see the sections "Using Extended ping Tests" and "CSU and DSU Loopback Tests," later in this chapter.
3. Look for patterns. For example, if errors occur at a consistent interval, they could be related to a periodic function, such as the sending of routing updates.
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Serial Lines: Troubleshooting Serial Line Input Errors
Table 15-5 describes the various types of input errors displayed by the show interfaces serial command (see Figure 15-1), possible problems that might be causing the errors, and solutions to those problems.
Table 15-5 Serial Lines: Troubleshooting Serial Line Input Errors
Input Error Type
(Field Name)
|
Possible Problem
|
Solution
|
CRC errors (CRC)
|
CRC errors occur when the CRC calculation does not pass (indicating that data is corrupted) for one of the following reasons:
•The serial line is noisy.
•The serial cable is too long, or the cable from the CSU/DSU to the router is not shielded
•SCTE mode is not enabled on DSU.
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1. Ensure that the line is clean enough for transmission requirements. Shield the cable, if necessary.
2. Make sure that the cable is within the recommended length (no more than 50 feet [15.24 meters], or 25 feet [7.62 meters] for a T1 link).
3. Ensure that all devices are properly configured for a common line clock. Set SCTE on the local and remote DSU. If your CSU/DSU does not support SCTE, see the section "Inverting the Transmit Clock," later in this chapter.
4. Make certain that the local and remote CSU/DSU are configured for the same framing and coding scheme as that used by the leased-line or other carrier service (for example, ESF/B8ZS).
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CRC errors (CRC) (continued)
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•The CSU line clock is incorrectly configured.
•A ones density problem has occurred on the T1 link (incorrect framing or coding specification).
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5. Contact your leased-line or other carrier service, and have it perform integrity tests on the line.
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Framing errors (frame)
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A framing error occurs when a packet does not end on an 8-bit byte boundary for one of the following reasons:
•The serial line is noisy
•The cable is improperly designed; the serial cable is too long; the cable from the CSU or DSU to the router is not shielded.
•SCTE mode is not enabled on the DSU; the CSU line clock is incorrectly configured; one of the clocks is configured for local clocking.
•A ones density problem has occurred on the T1 link (incorrect framing or coding specification).
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1. Ensure that the line is clean enough for transmission requirements. Shield the cable, if necessary. Make certain that you are using the correct cable.
2. Make sure that the cable is within the recommended length (no more than 50 feet [15.24 meters], or 25 feet [7.62 meters] for a T1 link).
3. Ensure that all devices are properly configured to use a common line clock. Set SCTE on the local and remote DSU. If your CSU/DSU does not support SCTE, see the section "Inverting the Transmit Clock," later in this chapter.
4. Make certain that the local and remote CSU/DSU is configured for the same framing and coding scheme as that used by the leased-line or other carrier service (for example, ESF1 /B8ZS2 ).
5. Contact your leased-line or other carrier service, and have it perform integrity tests on the line.
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Aborted transmission (abort)
|
Aborts indicate an illegal sequence of 1 bit (more than seven in a row)
The following are possible reasons for this to occur:
•SCTE mode is not enabled on DSU.
•The CSU line clock is incorrectly configured.
•The serial cable is too long, or the cable from the CSU or DSU to the router is not shielded.
•A ones density problem has occurred on the T1 link (incorrect framing or coding specification).
•A packet terminated in middle of transmission (typical cause is an interface reset or a framing error).
•A hardware problem has occurred (bad circuit, bad CSU/DSU, or bad sending interface on remote router).
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1. Ensure that all devices are properly configured to use a common line clock. Set SCTE on the local and remote DSU. If your CSU/DSU does not support SCTE, see the section "Inverting the Transmit Clock," later in this chapter.
2. Shield the cable, if necessary. Make certain that the cable is within the recommended length (no more than 50 feet [15.24 meters], or 25 feet [7.62 meters] for a T1 link). Ensure that all connections are good.
3. Check the hardware at both ends of the link. Swap faulty equipment, as necessary.
4. Lower data rates and determine whether aborts decrease.
5. Use local and remote loopback tests to determine where aborts are occurring (see the section "Special Serial Line Tests," later in this chapter).
6. Contact your leased-line or other carrier service, and have it perform integrity tests on the line.
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Serial Lines: Increasing Interface Resets on Serial Link
Interface resets that appear in the output of the show interfaces serial exec command (see Figure 15-1) are the result of missed keepalive packets.
Symptom: Increasing interface resets on serial link
Table 15-6 outlines the possible problems that might cause this symptom and describes solutions to those problems.
Table 15-6 Serial Lines: Increasing Interface Resets on Serial Link
Possible Problem
|
Solution
|
The following problems can result in this symptom:
•Congestion on link (typically associated with output drops)
•Bad line causing CD transitions
•Possible hardware problem at the CSU, DSU, or switch
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When interface resets are occurring, examine other fields of the show interfaces serial command output to determine the source of the problem. Assuming that an increase in interface resets is being recorded, examine the following fields:
1. If there is a high number of output drops in the show interfaces serial output, see the section "Serial Lines: Increasing Output Drops on Serial Link," earlier in this chapter.
2. Check the Carrier Transitions field in the show interfaces serial display. If carrier transitions are high while interface resets are being registered, the problem is likely to be a bad link or a bad CSU or DSU. Contact your leased-line or carrier service, and swap faulty equipment, as necessary.
3. Examine the Input Errors field in the show interfaces serial display. If input errors are high while interface resets are increasing, the problem is probably a bad link or a bad CSU/DSU. Contact your leased-line or other carrier service, and swap faulty equipment, as necessary.
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Serial Lines: Increasing Carrier Transitions Count on Serial Link
Carrier transitions appear in the output of the show interfaces serial exec command whenever there is an interruption in the carrier signal (such as an interface reset at the remote end of a link).
Symptom: Increasing carrier transitions count on serial link
Table 15-7 outlines the possible problems that might cause this symptom and describes solutions to those problems.
Table 15-7 Serial Lines: Increasing Carrier Transitions Count on Serial Link
Possible Problem
|
Solution
|
The following problems can result in this symptom:
•Line interruptions due to an external source (such as physical separation of cabling, red or yellow T1 alarms, or lightning striking somewhere along the network)
•Faulty switch, DSU, or router hardware
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1. Check hardware at both ends of the link (attach a breakout box or a serial analyzer, and test to determine the source of problems).
2. If an analyzer or breakout box is incapable of identifying any external problems, check the router hardware.
3. Swap faulty equipment, as necessary.
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Using the show controllers Command
The show controllers exec command is another important diagnostic tool when troubleshooting serial lines. The command syntax varies, depending on platform:
•For serial interfaces on Cisco 7000 series routers, use the show controllers cbus exec command.
•For Cisco access products, use the show controllers exec command.
•For the AGS, CGS, and MGS, use the show controllers mci exec command.
Figure 15-2 shows the output from the show controllers cbus exec command. This command is used on Cisco 7000 series routers with the Fast Serial Interface Processor (FSIP) card. Check the command output to make certain that the cable to the channel service unit/digital service unit (CSU/DSU) is attached to the proper interface. You can also check the microcode version to see whether it is current.
Figure 15-2 show controllers cbus Command Output
On access products such as the Cisco 2000, Cisco 2500, Cisco 3000, and Cisco 4000 series access servers and routers, use the show controllers exec command. Figure 15-3 shows the show controllers command output from the Basic Rate Interface (BRI) and serial interfaces on a Cisco 2503 access server. (Note that some output is not shown.)
The show controllers output indicates the state of the interface channels and whether a cable is attached to the interface. In Figure 15-3, serial interface 0 has an RS-232 DTE cable attached. Serial interface 1 has no cable attached.
Figure 15-4 shows the output of the show controllers mci command. This command is used on AGS, CGS, and MGS routers only. If the electrical interface is displayed as UNKNOWN (instead of V.35, EIA/TIA-449, or some other electrical interface type), an improperly connected cable is the likely problem. A bad applique or a problem with the internal wiring of the card is also possible. If the electrical interface is unknown, the corresponding display for the show interfaces serial exec command will show that the interface and line protocol are down.
Figure 15-3 show controllers Command Output
Figure 15-4 show controllers mci Command Output
Using debug Commands
The output of the various debug privileged exec commands provides diagnostic information relating to protocol status and network activity for many internetworking events.
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. When you finish using a
debug command, remember to disable it with its specific
no debug command or with the
no debug all command.
Following are some debug commands that are useful when troubleshooting serial and WAN problems. More information about the function and output of each of these commands is provided in the Debug Command Reference publication:
•debug serial interface—Verifies whether HDLC keepalive packets are incrementing. If they are not, a possible timing problem exists on the interface card or in the network.
•debug x25 events—Detects X.25 events, such as the opening and closing of switched virtual circuits (SVCs). The resulting cause and diagnostic information is included with the event report.
•debug lapb—Outputs Link Access Procedure, Balanced (LAPB) or Level 2 X.25 information.
•debug arp—Indicates whether the router is sending information about or learning about routers (with ARP packets) on the other side of the WAN cloud. Use this command when some nodes on a TCP/IP network are responding, but others are not.
•debug frame-relay lmi—Obtains Local Management Interface (LMI) information useful for determining whether a Frame Relay switch and a router are sending and receiving LMI packets.
•debug frame-relay events—Determines whether exchanges are occurring between a router and a Frame Relay switch.
•debug ppp negotiation—Shows Point-to-Point Protocol (PPP) packets transmitted during PPP startup, where PPP options are negotiated.
•debug ppp packet—Shows PPP packets being sent and received. This command displays low-level packet dumps.
•debug ppp errors—Shows PPP errors (such as illegal or malformed frames) associated with PPP connection negotiation and operation.
•debug ppp chap—Shows PPP Challenge Handshake Authentication Protocol (CHAP) and Password Authentication Protocol (PAP) packet exchanges.
•debug serial packet—Shows Switched Multimegabit Data Service (SMDS) packets being sent and received. This display also prints error messages to indicate why a packet was not sent or was received erroneously. For SMDS, the command dumps the entire SMDS header and some payload data when an SMDS packet is transmitted or received.
Using Extended ping Tests
The ping command is a useful test available on Cisco internetworking devices as well as on many host systems. In TCP/IP, this diagnostic tool is also known as an Internet Control Message Protocol (ICMP) echo request.
Note The ping command is particularly useful when high levels of input errors are being registered in the show interfaces serial display. See Figure 15-1.
Cisco internetworking devices provide a mechanism to automate the sending of many ping packets in sequence. Figure 15-5 illustrates the menu used to specify extended ping options. This example specifies 20 successive pings. However, when testing the components on your serial line, you should specify a much larger number, such as 1000 pings. Also increase the datagram size to a larger number, such as 1500.
Figure 15-5 Extended ping Specification Menu
In general, perform serial line ping tests as follows:
Step 1 Put the CSU or DSU into local loopback mode.
Step 2 Configure the extended ping command to send different data patterns and packet sizes. Figure 15-6 and Figure 15-7 illustrate two useful ping tests, an all-zeros 1500-byte ping and an all-ones 1500-byte ping, respectively.
Step 3 Examine the show interfaces serial command output (see Figure 15-1) and determine whether input errors have increased. If input errors have not increased, the local hardware (DSU, cable, router interface card) is probably in good condition.
Assuming that this test sequence was prompted by the appearance of a large number of CRC and framing errors, a clocking problem is likely. Check the CSU or DSU for a timing problem. See the section "Troubleshooting Clocking Problems," next.
Step 4 If you determine that the clocking configuration is correct and is operating properly, put the CSU or DSU into remote loopback mode.
Step 5 Repeat the ping test and look for changes in the input error statistics.
Step 6 If input errors increase, there is a problem either in the serial line or on the CSU/DSU. Contact the WAN service provider and swap the CSU or DSU. If problems persist, contact your technical support representative.
Figure 15-6 All-Zeros 1500-Byte ping Test
Figure 15-7 All-Ones 1500-Byte ping Test
Troubleshooting Clocking Problems
Clocking conflicts in serial connections can lead either to chronic loss of connection service or to degraded performance. This section discusses the important aspects of clocking problems: clocking problem causes, how to detect clocking problems, how to isolate clocking problems, and clocking problem solutions.
Clocking Overview
The CSU/DSU derives the data clock from the data that passes through it. To recover the clock, the CSU/DSU hardware must receive at least one 1-bit value for every 8 bits of data that pass through it; this is known as ones density. Maintaining ones density allows the hardware to recover the data clock reliably.
Newer T1 implementations commonly use Extended Superframe Format (ESF) framing with binary eight-zero substitution (B8ZS) coding. B8ZS provides a scheme by which a special code is substituted whenever eight consecutive zeros are sent through the serial link. This code is then interpreted at the remote end of the connection. This technique guarantees ones density independent of the data stream.
Older T1 implementations use D4 (also known as Superframe Format [SF]) framing and Alternate Mark Inversion (AMI) coding. AMI does not utilize a coding scheme like B8ZS. This restricts the type of data that can be transmitted because ones density is not maintained independent of the data stream.
Another important element in serial communications is serial clock transmit external (SCTE) terminal timing. SCTE is the clock echoed back from the data terminal equipment (DTE) device (for example, a router) to the data communications equipment (DCE) device (for example, the CSU/DSU).
When the DCE device uses SCTE instead of its internal clock to sample data from the DTE, it can better sample the data without error even if there is a phase shift in the cable between the CSU/DSU and the router. Using SCTE is highly recommended for serial transmissions faster than 64 kbps. If your CSU/DSU does not support SCTE, see the section "Inverting the Transmit Clock," later in this chapter.
Clocking Problem Causes
In general, clocking problems in serial WAN interconnections can be attributed to one of the following causes:
•Incorrect DSU configuration
•Incorrect CSU configuration
•Cables out of specification (longer than 50 feet [15.24 meters] or unshielded)
•Noisy or poor patch panel connections
•Several cables connected in a row
Detecting Clocking Problems
To detect clocking conflicts on a serial interface, look for input errors as follows:
Step 1 Use the show interfaces serial exec command on the routers at both ends of the link.
Step 2 Examine the command output for CRC, framing errors, and aborts.
Step 3 If either of these steps indicates errors exceeding an approximate range of 0.5 percent to 2.0 percent of traffic on the interface, clocking problems are likely to exist somewhere in the WAN.
Step 4 Isolate the source of the clocking conflicts, as outlined in the following section, "Isolating Clocking Problems."
Step 5 Bypass or repair any faulty patch panels.
Isolating Clocking Problems
After you determine that clocking conflicts are the most likely cause of input errors, use the following procedure to isolate the source of those errors:
Step 1 Perform a series of ping tests and loopback tests (both local and remote), as described in the section "CSU and DSU Loopback Tests," earlier in this chapter.
Step 2 Determine which end of the connection is the source of the problem, or whether the problem is in the line. In local loopback mode, run different patterns and sizes in the ping tests (for example, use 1500-byte datagrams). Using a single pattern and packet size may not force errors to materialize, particularly when a serial cable to the router or CSU/DSU is the problem.
Step 3 Use the show interfaces serial exec command, and determine whether input errors counts are increasing and where they are accumulating.
If input errors are accumulating on both ends of the connection, clocking of the CSU is the most likely problem.
If only one end is experiencing input errors, there is probably a DSU clocking or cabling problem.
Aborts on one end suggest that the other end is sending bad information or that there is a line problem.
Note Always refer to the show interfaces serial command output (see Figure 15-1). Log any changes in error counts, or note if the error count does not change.
Clocking Problem Solutions
Table 15-8 outlines suggested remedies for clocking problems, based on the source of the problem.
Table 15-8 Serial Lines: Clocking Problems and Solutions
Possible Problem
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Solution
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Incorrect CSU configuration
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1. Determine whether the CSUs at both ends agree on the clock source (local or line).
2. If the CSUs do not agree, configure them so that they do agree (usually the line is the source).
3. Check the LBO1 setting on the CSU to ensure that the impedance matches that of the physical line. For information on configuring your CSU, consult your CSU hardware documentation.
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Incorrect DSU configuration
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1. Determine whether the DSUs at both ends have SCTE mode enabled.
2. If SCTE is not enabled on both ends of the connection, enable it.
(For any interface that is connected to a line of 128 kbps or faster, SCTE must be enabled. If your DSU does not support SCTE, see the section "Inverting the Transmit Clock," later in this chapter.)
3. Make sure that ones density is maintained. This requires that the DSU use the same framing and coding schemes (for example, ESF and B8ZS) used by the leased-line or other carrier service.
Check with your leased-line provider for information on its framing and coding schemes.
4. If your carrier service uses AMI coding, either invert the transmit clock on both sides of the link, or run the DSU in bit-stuff mode. For information on configuring your DSU, consult your DSU hardware documentation.
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Cable to router out of specification
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If the cable is longer than 50 feet (15.24 meters), use a shorter cable.
If the cable is unshielded, replace it with shielded cable.
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Inverting the Transmit Clock
If you are attempting serial connections at speeds greater than 64 kbps with a CSU/DSU that does not support SCTE, you might have to invert the transmit clock on the router. Inverting the transmit clock compensates for phase shifts between the data and clock signals.
The specific command used to invert the transmit clock varies between platforms. On a Cisco 7000 series router, enter the invert-transmit-clock interface configuration command. For Cisco 4000 series routers, use the dte-invert-txc interface configuration command.
To ensure that you are using the correct command syntax for your router, refer to the user guide for your router or access server and to the Cisco IOS configuration guides and command references.
Note On older platforms, inverting the transmit clock might require that you move a physical jumper.
Adjusting Buffers
Excessively high bandwidth utilization greater than 70 percent results in reduced overall performance and can cause intermittent failures. For example, DECnet file transmissions might be failing because of packets being dropped somewhere in the network.
If the situation is bad enough, you must increase the bandwidth of the link. However, increasing the bandwidth might not be necessary or immediately practical. One way to resolve marginal serial line overutilization problems is to control how the router uses data buffers.
Caution In general, do
not adjust system buffers unless you are working closely with a Cisco technical support representative. You can severely affect the performance of your hardware and your network if you incorrectly adjust the system buffers on your router.
Use one of the following three options to control how buffers are used:
•Adjust parameters associated with system buffers.
•Specify the number of packets held in input or output queues (hold queues).
•Prioritize how traffic is queued for transmission (priority output queuing).
The configuration commands associated with these options are described in the Cisco IOS configuration guides and command references.
The following section focuses on identifying situations in which these options are likely to apply and defining how you can use these options to help resolve connectivity and performance problems in serial/WAN interconnections.
Tuning System Buffers
There are two general buffer types on Cisco routers: hardware buffers and system buffers. Only the system buffers are directly configurable by system administrators. The hardware buffers are specifically used as the receive and transmit buffers associated with each interface and (in the absence of any special configuration) are dynamically managed by the system software itself.
The system buffers are associated with the main system memory and are allocated to different-size memory blocks. A useful command for determining the status of your system buffers is the show buffers exec command. Figure 15-8 shows the output from the show buffers command.
Figure 15-8 show buffers Command Output
In the show buffers output, the following is true:
•total identifies the total number of buffers in the pool, including used and unused buffers.
•permanent identifies the permanent number of allocated buffers in the pool. These buffers are always in the pool and cannot be trimmed away.
•in free list identifies the number of buffers currently in the pool that are available for use.
•min identifies the minimum number of buffers that the route processor (RP) should attempt to keep in the free list:
–The min parameter is used to anticipate demand for buffers from the pool at any given time.
–If the number of buffers in the free list falls below the min value, the RP attempts to create more buffers for that pool.
•max allowed identifies the maximum number of buffers allowed in the free list:
–The max allowed parameter prevents a pool from monopolizing buffers that it doesn't need anymore, and frees this memory back to the system for further use.
–If the number of buffers in the free list is greater than the max allowed value, the RP should attempt to trim buffers from the pool.
•hits identifies the number of buffers that have been requested from the pool. The hits counter provides a mechanism for determining which pool must meet the highest demand for buffers.
•misses identifies the number of times that a buffer has been requested and that the RP detected that additional buffers were required. (In other words, the number of buffers in the free list has dropped below min.) The misses counter represents the number of times that the RP has been forced to create additional buffers.
•trims identifies the number of buffers that the RP has trimmed from the pool when the number of buffers in the free list exceeded the number of max allowed buffers.
•created identifies the number of buffers that has been created in the pool. The RP creates buffers when demand for buffers has increased until the number of buffers in the free list is less than min buffers or a miss occurs because of zero buffers in the free list.
•failures identifies the number of failures to grant a buffer to a requester even after attempting to create an additional buffer. The number of failures represents the number of packets that have been dropped due to buffer shortage.
•no memory identifies the number of failures caused by insufficient memory to create additional buffers.
The show buffers command output in Figure 15-8 indicates high numbers in the Trims and Created fields for large buffers. If you are receiving high numbers in these fields, you can increase your serial link performance by increasing the max free value configured for your system buffers. trims identifies the number of buffers that the RP has trimmed from the pool when the number of buffers in free list exceeded the number of max allowed buffers.
Use the buffers max free number global configuration command to increase the number of free system buffers. The value that you configure should be approximately 150 percent of the figure indicated in the total field of the show buffers command output. Repeat this process until the show buffers output no longer indicates trims and created buffers.
If the show buffers command output shows a large number of failures in the (no memory) field (see the last line of output in Figure 15-8), you must reduce the usage of the system buffers or increase the amount of shared or main memory (physical RAM) on the router. Call your technical support representative for assistance.
Implementing Hold Queue Limits
Hold queues are buffers used by each router interface to store outgoing or incoming packets. Use the hold-queue interface configuration command to increase the number of data packets queued before the router will drop packets. Increase these queues by small increments (for instance, 25 percent) until you no longer see drops in the show interfaces output. The default output hold queue limit is 100 packets.
Note The hold-queue command is used for process-switched packets and periodic updates generated by the router.
Use the hold-queue command to prevent packets from being dropped and to improve serial link performance under the following conditions:
•You have an application that cannot tolerate drops, and the protocol is capable of tolerating longer delays. DECnet is an example of a protocol that meets both criteria. Local-area transport (LAT) does not meet this criteria because it does not tolerate delays.
•The interface is very slow (bandwidth is low or anticipated utilization is likely to sporadically exceed available bandwidth).
Note When you increase the number specified for an output hold queue, you might need to increase the number of system buffers. The value used depends on the size of the packets associated with the traffic anticipated for the network.
Using Priority Queuing to Reduce Bottlenecks
Priority queuing is a list-based control mechanism that allows traffic to be prioritized on an interface-by-interface basis. Priority queuing involves two steps:
Step 1 Create a priority list by protocol type and level of priority.
Step 2 Assign the priority list to a specific interface.
Both of these steps use versions of the priority-list global configuration command. In addition, further traffic control can be applied by referencing access-list global configuration commands from priority-list specifications. For examples of defining priority lists and for details about command syntax associated with priority queuing, refer to the Cisco IOS configuration guides and command references.
Note Priority queuing automatically creates four hold queues of varying size. This overrides any hold queue specification included in your configuration.
Use priority queuing to prevent packets from being dropped and to improve serial link performance under the following conditions:
•When the interface is slow, a variety of traffic types are being transmitted, and you want to improve terminal traffic performance
•If you have a serial link that is intermittently experiencing very heavy loads (such as file transfers occurring at specific times), and priority queuing will help select which types of traffic should be discarded at high traffic periods
In general, start with the default number of queues when implementing priority queues. After enabling priority queuing, monitor output drops with the show interfaces serial exec command. If you notice that output drops are occurring in the traffic queue that you have specified to be high priority, increase the number of packets that can be queued (using the queue-limit keyword option of the priority-list global configuration command). The default queue-limit arguments are 20 packets for the high-priority queue, 40 for medium, 60 for normal, and 80 for low.
Note When bridging Digital Equipment Corporation (Digital) LAT traffic, the router must drop very few packets, or LAT sessions can terminate unexpectedly. A high-priority queue depth of about 100 (specified with the queue-limit keyword) is a typical working value when your router is dropping output packets and the serial lines are subjected to about 50 percent bandwidth utilization. If the router is dropping packets and is at 100 percent utilization, you need another line.
Another tool to relieve congestion when bridging Digital LAT is LAT compression. You can implement LAT compression with the interface configuration command bridge-group group lat-compression.
Special Serial Line Tests
In addition to the basic diagnostic capabilities available on routers, a variety of supplemental tools and techniques can be used to determine the conditions of cables, switching equipment, modems, hosts, and remote internetworking hardware. For more information, consult the documentation for your CSU, DSU, serial analyzer, or other equipment.
CSU and DSU Loopback Tests
If the output of the show interfaces serial exec command indicates that the serial line is up but the line protocol is down, use the CSU/DSU loopback tests to determine the source of the problem. Perform the local loop test first, and then perform the remote test. Figure 15-9 illustrates the basic topology of the CSU/DSU local and remote loopback tests.
Figure 15-9 CSU/DSU Local and Remote Loopback Tests
Note These tests are generic in nature and assume attachment of the internetworking system to a CSU or DSU. However, the tests are essentially the same for attachment to a multiplexer with built-in CSU/DSU functionality. Because there is no concept of a loopback in X.25 or Frame Relay packet-switched network (PSN) environments, loopback tests do not apply to X.25 and Frame Relay networks.
CSU and DSU Local Loopback Tests for HDLC or PPP Links
Following is a general procedure for performing loopback tests in conjunction with built-in system diagnostic capabilities:
Step 1 Place the CSU/DSU in local loop mode (refer to your vendor documentation). In local loop mode, the use of the line clock (from the T1 service) is terminated, and the DSU is forced to use the local clock.
Step 2 Use the show interfaces serial exec command to determine whether the line status changes from "line protocol is down" to "line protocol is up (looped)," or whether it remains down.
Step 3 If the line protocol comes up when the CSU or DSU is in local loopback mode, this suggests that the problem is occurring on the remote end of the serial connection. If the status line does not change state, there is a possible problem in the router, connecting cable, or CSU/DSU.
Step 4 If the problem appears to be local, use the debug serial interface privileged exec command.
Step 5 Take the CSU/DSU out of local loop mode. When the line protocol is down, the debug serial interface command output will indicate that keepalive counters are not incrementing.
Step 6 Place the CSU/DSU in local loop mode again. This should cause the keepalive packets to begin to increment. Specifically, the values for mineseen and yourseen keepalives will increment every 10 seconds. This information will appear in the debug serial interface output.
If the keepalives do not increment, there may be a timing problem on the interface card or on the network. For information on correcting timing problems, refer to the section "Troubleshooting Clocking Problems," earlier in this chapter.
Step 7
