What is a serial interface? A serial interface is a communication interface between two digital systems that transmits data as a series of voltage pulses down a wire. A '1' is represented by a high logical voltage and a '0' is represented by a low logical voltage. Essentially, the serial interface encodes the bits of a binary number by their. Configuring the Serial Interface. The Cisco 800M Series Integrated Services Router (ISR) provides serial WAN connectivity to remote sites.
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Table Of ContentsTroubleshooting Serial LinesThis 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 ProblemsTroubleshooting Using the show interfaces serial CommandThe 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. SolutionSerial x is up, line protocol is up—This is the proper status line condition.
No action is required.Serial x is down, line protocol is down (DTE mode)The router is not sensing a CD signal (that is, the CD is not active).A telephone company problem has occurred—line is down or is not connected to CSU /DSU.Cabling is faulty or incorrect.Hardware failure has occurred (CSU/DSU).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.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 (SCTE 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 (DCE 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 bpsSyntax 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.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.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.Serial x is up, line protocol is down (disabled)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.1.
Troubleshoot the line with a serial analyzer and breakout box. Look for toggling CTS and DSR 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).Serial x is administrat-ively down, line protocol is downThe router configuration includes the shutdown interface configuration command.A duplicate IP address exists.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. SolutionInput rate to serial interface exceeds bandwidth available on serial link1. Minimize periodic broadcast traffic, such as routing and SAP 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.Input rate to serial interface exceeds bandwidth available on serial link (continued)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 IPX ). SolutionInput rate exceeds the capacity of the router, or input queues exceed the size of output queuesNote: Input drop problems are typically seen when traffic is being routed between faster interfaces (such as Ethernet, Token Ring, and FDDI ) 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.Input rate exceeds the capacity of the router, or input queues exceed the size of output queues (continued)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.FDDI = Fiber Distributed Data InterfaceSerial Lines: Increasing Input Errors in Excess of 1 Percent of Total Interface TrafficIf 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.
SolutionThe 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 DSU2. 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.Serial Lines: Troubleshooting Serial Line Input ErrorsTable 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. SolutionCRC 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.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).CRC errors (CRC) (continued). The CSU line clock is incorrectly configured. A ones density problem has occurred on the T1 link (incorrect framing or coding specification).5. SolutionThe 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 switchWhen 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.Serial Lines: Increasing Carrier Transitions Count on Serial LinkCarrier 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 linkTable 15-7 outlines the possible problems that might cause this symptom and describes solutions to those problems.
SolutionThe 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 hardware1.
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.Using the show controllers CommandThe 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 15-2show 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.
Serial Interface Software
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 15-3show controllers Command Output. 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 TestsThe 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 15-5Extended 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 15-6All-Zeros 1500-Byte ping Test. Troubleshooting Clocking ProblemsClocking 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 OverviewThe 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. SolutionIncorrect CSU configuration1.
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 LBO 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.Incorrect DSU configuration1. 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.Cable to router out of specificationIf the cable is longer than 50 feet (15.24 meters), use a shorter cable.If the cable is unshielded, replace it with shielded cable.LBO = line build outInverting the Transmit ClockIf 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 BuffersExcessively 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. 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.
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 BottlenecksPriority 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. 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 TestsIn 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 TestsIf 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 15-9CSU/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 LinksFollowing 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 Check the local router and CSU/DSU hardware, and any attached cables.
Make certain that the cables are within the recommended lengths (no more than 50 feet 15.24 meters, or 25 feet 7.62 meters for a T1 link). Make certain that the cables are attached to the proper ports. Swap faulty equipment, as necessary.Figure 15-10 shows the output from the debug serial interface command for an HDLC serial connection, with missed keepalives causing the line to go down and the interface to reset.Figure 15-10debug serial interface Command Output. Note This remote loopback test assumes that HDLC encapsulation is being used and that the preceding local loop test was performed immediately before this test.The following are the steps required to perform loopback testing:Step 1 Put the remote CSU or DSU into remote loopback mode (refer to the vendor documentation).Step 2 Using the show interfaces serial exec command, determine whether the line protocol remains up, with the status line indicating 'Serial x is up, line protocol is up (looped),' or goes down, with the status line indicating 'line protocol is down.'
Step 3 If the line protocol remains up (looped), the problem is probably at the remote end of the serial connection (between the remote CSU/DSU and the remote router). Perform both local and remote tests at the remote end to isolate the problem source.Step 4 If the line status changes to 'line protocol is down' when remote loopback mode is activated, make certain that ones density is being properly maintained. Step 4 Try using a rollover cable.Run the show controller t1 exec command after each step to see whether the controller exhibits any errors.If the Line Is in Loopback ModeCheck to see whether the line is in loopback mode from the show controller t1 output. A line should be in loopback mode only for testing purposes.To turn off loopback, use the no loopback command in the controller configuration mode, as shown here:maui-nas-03(config-controlle)# no loopbackIf the Controller Displays Any AlarmsCheck the show controller command output to see if there are alarms displayed by the controller.We will now discuss various alarms and the procedure necessary to correct them.Receive (RX) Alarm Indication Signal (AIS) (Blue)A received alarm indication signal (AIS) means that an alarm is occurring on the line upstream of the equipment connected to the port. The AIS failure is declared when an AIS defect is detected at the input and still exists after the loss of frame failure is declared (caused by the unframed nature of the 'all-ones' signal). The AIS failure is cleared when the loss of frame failure is cleared.Step 1 Check to see whether the framing format configured on the port matches the framing format of the line.
If not, change the framing format on the controller to match that of the line.Step 2 Contact your service provider to check for misconfiguration within the telco.Receive (Rx) Remote Alarm Indication (Yellow)A received remote alarm indication means that the far-end equipment has a problem with the signal that it is receiving from its upstream equipment.For SF links, the far-end alarm failure is declared when bit 6 of all the channels has been zero for at least 335 ms. The failure is cleared when bit 6 of at least one channel is not zero for a period usually less than 1 second and always less than 5 seconds. The far-end alarm failure is not declared for SF links when a loss of signal is detected.For ESF links, the far-end alarm failure is declared if the yellow alarm signal pattern occurs in at least seven out of ten contiguous 16-bit pattern intervals. The failure is cleared if the yellow alarm signal pattern does not occur in ten contiguous 16-bit signal pattern intervals.Step 1 Insert an external loopback cable into the port. To create a loopback plug, refer to the section 'Performing Hardware Loopback Plug Test,' later in this chapter.Step 2 Check to see if there are any alarms.
If you do not see any alarms, then the local hardware is probably in good condition. In that case, do the following:. Check the cabling. Refer to the section 'If Receiver Has Loss of Signal' for more information. Check the settings at the remote end, and verify that they match your port settings.
If the problem persists, contact your service provider.Step 3 Remove the loopback plug, and reconnect your T1 lineStep 4 Check the cabling. Refer to the section 'Loss of Signal' for more information.Step 5 Power-cycle the router.Step 6 Connect the T1 line to a different port. Configure the port with the same settings as that of the line. If the problem does not persist, then the fault lies with the one port:.
Reconnect the T1 line to the original port. Proceed to the 'Troubleshooting Error Events' section, later in this chapter.If the problem persists, then do the following:. Perform a hardware loop test, as described in the section 'Performing Hardware Loopback Plug Test.' . Replace the T1 controller card. Proceed to 'Troubleshooting Error Events,' the next section.Transmitter Sending Remote Alarm (Red)A red alarm is declared when the CSU cannot synchronize with the framing pattern on the T1 line.Step 1 Check to see whether the framing format configured on the port matches the framing format of the line. Maui-nas-03(config-if)# no loopbackStep 5 Power-cycle the router.Step 6 If the problem persists, contact your service provider or Cisco TAC.Troubleshooting a PRIWhenever troubleshooting a PRI, you need to check whether the T1 is running cleanly on both ends.
If Layer 1 problems have been resolved, as described previously, we must look to Layer 2 and 3 problems.Troubleshooting Using the show isdn status CommandThe show isdn status command is used to display a snapshot of all ISDN interfaces. It displays the status of Layers 1, 2, and 3.Step 1 Verify that Layer 1 is active.The Layer 1 status should always say ACTIVE unless the T1 is down.If show isdn status indicates that Layer 1 is DEACTIVATED, then there is a problem with the physical connectivity on the T1 line. Refer to the previous section 'Is the Controller Administratively Down?' Also verify that the T1 is not administratively down. Use the no shutdown command to bring up the T1 controller.Step 2 Check whether Layer 2 state is MULTIPLEFRAMEESTABLISHED.The desired Layer 2 State is MULTIPLEFRAMEESTABLISHED, which indicates that we are exchanging Layer 2 frames and have finished Layer 2 initialization.If Layer 2 is not MULTIPLEFRAMEESTABLISHED, use the show controller t1 exec command to diagnose the problem. Refer to the section 'Troubleshooting Using the show controller t1 Command.'
Because show isdn status is a snapshot of the current status, it is possible that Layer 2 is bouncing up and down despite indicating MULTIPLEFRAMEESTABLISHED. Use debug isdn q921 to verify that Layer 2 is stable.Using debug q921The debug isdn q921 command displays data link layer (Layer 2) access procedures that are taking place at the router on the D-channel.Ensure that you are configured to view debug messages by using the logging console or terminal monitor command as necessary. Step 7 Power-cycle the router.Step 8 If the problem persists, contact your service provider or Cisco TAC.Performing Hardware Loopback Plug TestThe hardware loopback plug test can be used to test whether the router has any faults. If a router passes a hardware loopback plug test, then the problem exists elsewhere on the line.To create a loopback plug, follow these steps:Step 1 Use wire cutters to cut a working RJ-45 or RJ-48 cable so that there are 5 inches of cable and the connector attached to it.Step 2 Strip the wires.Step 3 Twist the wires from pins 1 and 4 together.Step 4 Twist the wires from pins 2 and 5 together.Leave the rest of the wires alone.The pins on an RJ-45/48 jack are numbered from 1 through 8.
Pin 1 is the left-most pin when looking at the jack with the metal pins facing you.Performing the Loopback Plug TestStep 1 Insert the plug into the T1 port in question.Step 2 Save your router configuration using the write memory command. Step 6 Perform the extended ping test as described in the 'Using Extended ping Tests' section, earlier in this chapter.Troubleshooting E1 ProblemsThis section describes the techniques and procedures to troubleshoot E1 circuits for dial-in customers.Troubleshooting Using the show controller e1 CommandThe show e1 controller exec command provides information to logically troubleshoot physical layer and data link layer problems. This section describes how to logically troubleshoot using the show controller e1 command.This command displays the controller status that is specific to the controller hardware.
The information displayed is generally useful for diagnostic tasks performed by technical support personnel only.The NPM or MIP can query the port adapters to determine their current status. Issue a show controller e1 command to display statistics about the E1 link.If you specify a slot and port number, statistics for each 15-minute period will be displayed.Most E1 errors are caused by misconfigured lines. Ensure that linecoding, framing, clock source, and line termination (balanced or unbalanced) are configured according to what the service provider recommended.Show controller e1 ConditionsThe E1 controller can be in three states:.
Administratively down. Down. UpIs the Controller Administratively Down?The controller is administratively down when it has been manually shut down. You should restart the controller to correct this error.Step 1 Enter enable mode.
Step 2 Try the other framing format to see if the alarm clears.If this does not fix the problem, proceed to the receiver has loss of signal section below.Step 3 Check the framing format on the remote end.Step 4 Check the linecoding on the remote end.If Receiver Has Loss of SignalStep 1 Make sure that the cable between the interface port and the E1 service provider's equipment or E1 terminal equipment is connected correctly. Check to see whether the cable is hooked up to the correct ports. Correct the cable connections if necessary.Step 2 Check cable integrity. Look for breaks or other physical abnormalities in the cable. Ensure that the pinouts are set correctly. If necessary, replace the cable.Step 3 Check the cable connectors.
A reversal of the transmit and receive pairs or an open receive pair can cause errors. Set the receive pair to lines 1 and 2; the transmit pair should be lines 4 and 5.The pins on a RJ-48 jack are numbered from 1 through 8. Pin 1 is the leftmost pin when looking at the jack with the metal pins facing you. Refer to Figure 15-12 for more information.Figure 15-12RJ-45 Cable. Step 4 Try using a rollover cable.Step 5 Check to see whether there are far-end block errors.
If so, the problem exists with the receive lead on the local end. Contact TAC for more assistance.Run the show controller e1 exec command after each step to check whether the controller exhibits any errors.If the Line Is in Loopback ModeCheck to see whether the line is in loopback mode from the show controller e1 output. Maui-nas-03(config-controlle)# linecode amiVerifying That isdn switchtype and pri-group Are Configured CorrectlyUse the show running-config command to check whether isdn switchtype and pri-group timeslots are configured correctly. Contact your service provider for correct values.To change the isdn switchtype and pri-group, use these lines:maui-nas-03# configure terminalmaui-nas-03(config)# isdn switch-type primary-net5maui-nas-03(config)# controller e1 0maui-nas-03(config-controlle)# pri-group timeslots 1-31Verifying the Signaling ChannelIf the error counters do not increase but the problem persists, verify that the signaling channel is up and configured correctly.Step 1 Run the show interface serial x:15 command, where x should be replaced by the interface number.Step 2 Check to see whether the interface is up. If the interface is not up, use the no shutdown command to bring up the interface.
Step 5 Power-cycle the router.Step 6 If the problem persists, contact your service provider or Cisco TAC.Troubleshooting a PRIWhenever troubleshooting a PRI, you need to check whether the E1 is running cleanly on both ends. If Layer 1 problems have been resolved, as described previously, we must look to Layer 2 and 3 problems.Troubleshooting Using the show isdn status CommandThe show isdn status command is used to display a snapshot of all ISDN interfaces. It displays the status of Layers 1, 2, and 3.Step 1 Verify that Layer 1 is active.The Layer 1 status should always say ACTIVE unless the E1 is down.If show isdn status indicates that Layer 1 is DEACTIVATED, then there is a problem with the physical connectivity on the E1 line. Refer to the section 'Is the Controller Administratively Down?'
Also verify that the E1 is not administratively down. Use the no shutdown command to bring up the E1 controller.Step 2 Check whether Layer 2 state is MULTIPLEFRAMEESTABLISHED.The desired Layer 2 state is MULTIPLEFRAMEESTABLISHED, which indicates that the startup protocol between ISDN switch and end device has been established and that we are exchanging Layer 2 frames.If Layer 2 is not MULTIPLEFRAMEESTABLISHED, use the show controller E1 exec command to diagnose the problem. Refer to the previous section 'Troubleshooting Using the show controller e1 Command,' and the upcoming section 'Troubleshooting E1 Error Events.'
Because show isdn status is a snapshot of the current status, it is possible that Layer 2 is bouncing up and down despite indicating MulitpleFrameEstablished. Use debug isdn q921 to verify that Layer 2 is stable.Using debug q921The debug isdn q921 command displays data link layer (Layer 2) access procedures that are taking place at the router on the D-channel.Ensure that you are configured to view debug messages by using the logging console or terminal monitor commands, as necessary.