The BrightSpark's Home - CCNA 640-801 Gateway

BSIG: Fast Notes - Trouble-shooting Serial Interfaces

Serial line states:

Serial x is up, line protocol is up 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

The following table offers explanations on these serial line states:

Status Line Line State Resolution

Serial 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 up (DTE mode) The route is not sensing a CD (Carrier Detect) signal. CD is inactive.
A telephone company problem has occurred. The line is down or is not connected to the CSU/DSU.
The cabling is faulty or incorrect.
Hardware failure has occurred within the 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 (Check your hardware installation documentation).
3) Insert a breakout box and check all control leads.
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.
The remote router is not sending keepalives.
A leased-line or other carrier service problem has occurred (noisy line, misconfigured, or failed switch).
A timing problem has occurred on the cable. (SCTE is not set on the 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, the telephone company or a failed remote router is the likely cause of the problem.
2) If the problem appears to be on the remote end, repeat Step 1 on the remote modem, CSU, or DSU.
Verify all cabling. Make certain that the cable is attached to the correct interface, the correct CSU/DSU, or network termination point. Use the "show controllers" to determine which cable is attached to which interface.
4) Enable the "debug serial interface" command.
5) If the line protocol does not come up in local loopback mode, and the output of the "debug serial interface" command shows that the keepalive counter is not incrementing, a router hardware problem is likely. Swap out router interface hardware.
6) If the the line protocol comes up and the keepalive counter increments, the problem is not in the local router.
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 up (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.
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:
"clockrate x bps"
bps is the desired clock rate in bits per second: 1200, 2400, 2800, 9600, 19200, 38400, 56000, 64000, 72000, 125,000, 148000, 250000, 500000, 800000, 1000000, 1300000, 2000000, 4000000, 8000000.
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 breakdout box and observe leads.
5) Replace faulty parts, as necessary.

Serial x is up, line protocol is up (looped) A loop exisits in the circuit. The sequence number in the keepalive packet changes to a random number when a loop in initially detected. If the same random number is returned over the link, a loop exists. 1) Use the "show running-config" command to look for any "loopback" interface configuration command entries.
2) If you find a "loopback" interface configuration comand enty, use the "no loopback" interface configuration command to remove the loop.
3) If you do not find the "loopack" 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 trouble-shooting 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) Trouble-shoot the line with a serial analyzer and breakout box. Look for toggling "Clear to Send" and "Data Set Ready" 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 administratively down, line protocol is down The router configuration includes the "shutdown" interface configuration command.
A duplicate IP address exists.
1) Use the "show running-config" command to 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.
4) If there are duplicate addresses, resolve the conflct by changing one of the IP addresses.

A couple of questions ....?
How do you know which side of the circuit is DTE or DCE? (Check out the third line within the output of both command examples below)

LabB# show contollers serial 0/0 Interface Serial0/0 Hardware is PowerQUICC MPC860 DCE V.35, clock rate 56000 idb at 0x80C0E718, driver data structure at 0x80C14674 SCC Registers: General [GSMR]=0x2:0x00000030, Protocol-specific [PSMR]=0x8 Events [SCCE]=0x0000, Mask [SCCM]=0x001F, Status [SCCS]=0x06 Transmit on Demand [TODR]=0x0, Data Sync [DSR]=0x7E7E Interrupt Registers: Config [CICR]=0x00367F80, Pending [CIPR]=0x00008000 Mask [CIMR]=0x48204002, In-srv [CISR]=0x00000000 Command register [CR]=0x6C0 Port A [PADIR]=0x1100, [PAPAR]=0xFFFF [PAODR]=0x0000, [PADAT]=0xE6FF Port B [PBDIR]=0x09C0F, [PBPAR]=0x0600E [PBODR]=0x0000E, [PBDAT]=0x3037D Port C [PCDIR]=0x00C, [PCPAR]=0x000 [PCSO]=0x820, [PCDAT]=0xFC0, [PCINT]=0x00F Receive Ring rmd(68012830): status 9000 length 1A address 1B6D044 rmd(68012838): status 9000 length 1A address 1B6C9C4 rmd(68012840): status 9000 length 1A address 1B71144 rmd(68012848): status 9000 length 1A address 1B72B44 rmd(68012848): status 9000 length 1A address 1B72B44 rmd(68012850): status 9000 length 11E address 1B72B44 rmd(68012858): status 9000 length 1A address 1B71E44 rmd(68012860): status 9000 length 11E address 1B717C4 rmd(68012868): status 9000 length 1A address 1B70AC4 rmd(68012870): status 9000 length 1A address 1B70444 rmd(68012878): status 9000 length 1A address 1B6FDC4 rmd(68012880): status 9000 length 1A address 1B6F744 rmd(68012888): status 9000 length 1A address 1B6F0C4 rmd(68012890): status 9000 length 1A address 1B6EA44 rmd(68012898): status 9000 length 11E address 1B6E3C4 rmd(680128A0): status 9000 length 1A address 1B6DD44 rmd(680128A8): status B000 length 1A address 1B6D6C4 Transmit Ring tmd(680128B0): status 5C00 length 18 address 1B80C54 tmd(680128B8): status 5C00 length 44 address 1B80B14 tmd(680128C0): status 5C00 length 18 address 1B80ED4 tmd(680128C8): status 5C00 length 44 address 1B80B14 tmd(680128D0): status 5C00 length 18 address 1B80ED4 tmd(680128D8): status 5C00 length 38 address 1B824D4 tmd(680128E0): status 5C00 length 44 address 1B80ED4 tmd(680128E8): status 5C00 length 18 address 1B80ED4 tmd(680128F0): status 5C00 length 44 address 1B809D4 tmd(680128F8): status 5C00 length 18 address 1B80ED4 tmd(68012900): status 5C00 length 44 address 1B809D4 tmd(68012908): status 5C00 length 18 address 1B80ED4 tmd(68012910): status 5C00 length 38 address 1B824D4 tmd(68012918): status 5C00 length 44 address 1B80B14 tmd(68012920): status 5C00 length 18 address 1B809D4 tmd(68012928): status 7C00 length 44 address 1B80B14 SCC GENERAL PARAMETER RAM (at 0x68013C00) Rx BD Base [RBASE]=0x2830, Fn Code [RFCR]=0x18 Tx BD Base [TBASE]=0x28B0, Fn Code [TFCR]=0x18 Max Rx Buff Len [MRBLR]=1548 Rx State [RSTATE]=0x18008440, BD Ptr [RBPTR]=0x2868 Tx State [TSTATE]=0x18000548, BD Ptr [TBPTR]=0x28B8 SCC HDLC PARAMETER RAM (at 0x68013C38) CRC Preset [C_PRES]=0xFFFF, Mask [C_MASK]=0xF0B8 Errors: CRC [CRCEC]=0, Aborts [ABTSC]=0, Discards [DISFC]=0 Nonmatch Addr Cntr [NMARC]=0 Retry Count [RETRC]=0 Max Frame Length [MFLR]=1608 Rx Int Threshold [RFTHR]=0, Frame Cnt [RFCNT]=60569 User-defined Address 0000/0000/0000/0000 User-defined Address Mask 0x0000 Tx State [TSTATE]=0x18000548, BD Ptr [TBPTR]=0x28B8 SCC HDLC PARAMETER RAM (at 0x68013C38) CRC Preset [C_PRES]=0xFFFF, Mask [C_MASK]=0xF0B8 Errors: CRC [CRCEC]=0, Aborts [ABTSC]=0, Discards [DISFC]=0 Nonmatch Addr Cntr [NMARC]=0 Retry Count [RETRC]=0 Max Frame Length [MFLR]=1608 Rx Int Threshold [RFTHR]=0, Frame Cnt [RFCNT]=60569 User-defined Address 0000/0000/0000/0000 User-defined Address Mask 0x0000 buffer size 1524 PowerQUICC SCC specific errors: 0 input aborts on receiving flag sequence 0 throttles, 0 enables 0 overruns 0 transmitter underruns 0 transmitter CTS losts LabB#

LabC# show controllers serial 0/0 Interface Serial0/0 Hardware is PowerQUICC MPC860 DTE V.35 TX and RX clocks detected. idb at 0x80C0E718, driver data structure at 0x80C14674 SCC Registers: General [GSMR]=0x2:0x00000030, Protocol-specific [PSMR]=0x8 Events [SCCE]=0x0000, Mask [SCCM]=0x001F, Status [SCCS]=0x06 Transmit on Demand [TODR]=0x0, Data Sync [DSR]=0x7E7E Interrupt Registers: Config [CICR]=0x00367F80, Pending [CIPR]=0x00008000 Mask [CIMR]=0x48204002, In-srv [CISR]=0x00000000 Command register [CR]=0x6C0 Port A [PADIR]=0x1000, [PAPAR]=0xFFFF [PAODR]=0x0000, [PADAT]=0xE6FF Port B [PBDIR]=0x09C0F, [PBPAR]=0x0600E [PBODR]=0x0000E, [PBDAT]=0x3037D Port C [PCDIR]=0x00C, [PCPAR]=0x000 [PCSO]=0x820, [PCDAT]=0xFC0, [PCINT]=0x00F Receive Ring rmd(68012830): status 9000 length 2F address 1B6FDC4 rmd(68012838): status 9000 length 2F address 1B6F744 rmd(68012840): status 9000 length 2F address 1B6F0C4 rmd(68012848): status 9000 length 2F address 1B6EA44 rmd(68012850): status 9000 length 1A address 1B6F0C4 rmd(68012858): status 9000 length 3A address 1B6EA44 rmd(68012860): status 9000 length 46 address 1B6E3C4 rmd(68012868): status 9000 length 1A address 1B6DD44 rmd(68012870): status 9000 length 46 address 1B6D6C4 rmd(68012878): status 9000 length 1A address 1B6D044 rmd(68012880): status 9000 length 2F address 1B6C9C4 rmd(68012888): status 9000 length 46 address 1B72B44 rmd(68012890): status 9000 length 1A address 1B724C4 rmd(68012898): status 9000 length 3A address 1B71E44 rmd(680128A0): status 9000 length 46 address 1B717C4 rmd(680128A8): status B000 length 1A address 1B71144 Transmit Ring tmd(680128B0): status 5C00 length 25C address 1B824D4 tmd(680128B8): status 5C00 length 1DA address 1B82B54 tmd(680128C0): status 5C00 length 18 address 1B80B14 tmd(680128C8): status 5C00 length 18 address 1B80D94 tmd(680128D0): status 5C00 length 11C address 1B95D94 tmd(680128D8): status 5C00 length 18 address 1B80B14 tmd(680128E0): status 5C00 length 18 address 1B80D94 tmd(680128E8): status 5C00 length 18 address 1B80D94 tmd(680128F0): status 5C00 length 18 address 1B80D94 tmd(680128F8): status 5C00 length 18 address 1B80D94 tmd(68012900): status 5C00 length 18 address 1B80D94 tmd(68012908): status 5C00 length 11C address 1B95D94 tmd(68012910): status 5C00 length 18 address 1B80B14 tmd(68012918): status 5C00 length 18 address 1B80D94 tmd(68012920): status 5C00 length 18 address 1B80D94 tmd(68012928): status 7C00 length 25C address 1B82B54 SCC GENERAL PARAMETER RAM (at 0x68013C00) Rx BD Base [RBASE]=0x2830, Fn Code [RFCR]=0x18 Tx BD Base [TBASE]=0x28B0, Fn Code [TFCR]=0x18 Max Rx Buff Len [MRBLR]=1548 Rx State [RSTATE]=0x18008440, BD Ptr [RBPTR]=0x2848 Tx State [TSTATE]=0x18800511, BD Ptr [TBPTR]=0x28D0 SCC HDLC PARAMETER RAM (at 0x68013C38) CRC Preset [C_PRES]=0xFFFF, Mask [C_MASK]=0xF0B8 Errors: CRC [CRCEC]=0, Aborts [ABTSC]=0, Discards [DISFC]=0 Nonmatch Addr Cntr [NMARC]=0 Retry Count [RETRC]=0 Max Frame Length [MFLR]=1608 Rx Int Threshold [RFTHR]=0, Frame Cnt [RFCNT]=51421 User-defined Address 0000/0000/0000/0000 User-defined Address Mask 0x0000 Tx State [TSTATE]=0x18800511, BD Ptr [TBPTR]=0x28D0 SCC HDLC PARAMETER RAM (at 0x68013C38) CRC Preset [C_PRES]=0xFFFF, Mask [C_MASK]=0xF0B8 Errors: CRC [CRCEC]=0, Aborts [ABTSC]=0, Discards [DISFC]=0 Nonmatch Addr Cntr [NMARC]=0 Retry Count [RETRC]=0 Max Frame Length [MFLR]=1608 Rx Int Threshold [RFTHR]=0, Frame Cnt [RFCNT]=51421 User-defined Address 0000/0000/0000/0000 User-defined Address Mask 0x0000 buffer size 1524 PowerQUICC SCC specific errors: 0 input aborts on receiving flag sequence 0 throttles, 0 enables 0 overruns 0 transmitter underruns 0 transmitter CTS losts LabC#

How do you verify the encapsulation type on a serial interface?

LabC# show interface serial 0/0 Serial0/0 is up, line protocol is up Hardware is PowerQUICC Serial Internet address is 192.168.10.2/28 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation HDLC, loopback not set Keepalive set (10 sec) Last input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters never Input queue: 0/75/0 (size/max/drops); Total output drops: 0 Queueing strategy: weighted fair Output queue: 0/1000/64/0 (size/max total/threshold/drops) Conversations 0/2/256 (active/max active/max total) Reserved Conversations 0/0 (allocated/max allocated) 5 minute input rate 0 bits/sec, 1 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 145501 packets input, 7481487 bytes, 0 no buffer Received 84716 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 70819 packets output, 4337241 bytes, 0 underruns 0 output errors, 0 collisions, 4 interface resets 0 output buffer failures, 0 output buffers swapped out 11 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up LabC#

Note the fifth line of output -- encapsulation = HDLC

LabC# show interface serial 0/0 Serial0/0 is up, line protocol is up Hardware is PowerQUICC Serial Internet address is 192.168.10.2/28 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation PPP, loopback not set, keepalive set (10 sec) LCP Open Open: IPCP, CDPCP Last input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters never ----->

Note the fifth line of output -- encapsulation = PPP. Also, notice the use of LCP - "Link Control Protocol".
Some important acronyms:

DCE (Data Circuit-Terminating Equipment). Devices and connections of a network that consist of the network end of the user-to-network interface. The DCE provides the physical connection to the network, forwards traffic and provides a clocking signal used to synchronize data transmission between DCE and DTE devices. Modems and interface cards are examples of DCE equipment.
DTE (Data Terminal Equipment). Device at the user end of a user-network interface that serves as a data source, destination, or both. DTE connects to a data network through a DCE device and typically uses clocking signal generated by the DCE. DTE includes such devices as computers, protocol translators and multiplexers.
CSU (Channel Service Unit). Digital interface device that connects end-user equipment to the local digital telephone loop.
DSU (Digital Service Unit) - more commonly known as a Data Service Unit. Device used in digital transmission that adapts the physical interface on a DTE device to a transmission facility such as T1 (US) or E1 (UK). The DSU is also responsible for such functions as signal timing.

It is very important that you understand the distinction between DTE and DCE.
DCE - apply the clock rate to the interface as in 56000 kbps. Always verify using "show running-config" for example. Again, if you do not know which side of the circuit is DCE or DTE -- "show controllers" will help you out.

Status Line	Line State	Resolution
Serial 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 up (DTE mode)	The route is not sensing a CD (Carrier Detect) signal. CD is inactive. A telephone company problem has occurred. The line is down or is not connected to the CSU/DSU. The cabling is faulty or incorrect. Hardware failure has occurred within the 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 (Check your hardware installation documentation). 3) Insert a breakout box and check all control leads. 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. The remote router is not sending keepalives. A leased-line or other carrier service problem has occurred (noisy line, misconfigured, or failed switch). A timing problem has occurred on the cable. (SCTE is not set on the 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, the telephone company or a failed remote router is the likely cause of the problem. 2) If the problem appears to be on the remote end, repeat Step 1 on the remote modem, CSU, or DSU. Verify all cabling. Make certain that the cable is attached to the correct interface, the correct CSU/DSU, or network termination point. Use the "show controllers" to determine which cable is attached to which interface. 4) Enable the "debug serial interface" command. 5) If the line protocol does not come up in local loopback mode, and the output of the "debug serial interface" command shows that the keepalive counter is not incrementing, a router hardware problem is likely. Swap out router interface hardware. 6) If the the line protocol comes up and the keepalive counter increments, the problem is not in the local router. 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 up (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. 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: "clockrate x bps" bps is the desired clock rate in bits per second: 1200, 2400, 2800, 9600, 19200, 38400, 56000, 64000, 72000, 125,000, 148000, 250000, 500000, 800000, 1000000, 1300000, 2000000, 4000000, 8000000. 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 breakdout box and observe leads. 5) Replace faulty parts, as necessary.
Serial x is up, line protocol is up (looped)	A loop exisits in the circuit. The sequence number in the keepalive packet changes to a random number when a loop in initially detected. If the same random number is returned over the link, a loop exists.	1) Use the "show running-config" command to look for any "loopback" interface configuration command entries. 2) If you find a "loopback" interface configuration comand enty, use the "no loopback" interface configuration command to remove the loop. 3) If you do not find the "loopack" 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 trouble-shooting 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) Trouble-shoot the line with a serial analyzer and breakout box. Look for toggling "Clear to Send" and "Data Set Ready" 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 administratively down, line protocol is down	The router configuration includes the "shutdown" interface configuration command. A duplicate IP address exists.	1) Use the "show running-config" command to 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. 4) If there are duplicate addresses, resolve the conflct by changing one of the IP addresses.