Serial Link Troubleshooting Commands


Several clear , show , and debug commands can be used to troubleshoot serial links. This section covers the following commands:

  • clear counters serial

  • debug serial interface

  • show buffers

  • show controllers serial

  • show interface serial

The clear counters serial Command

The clear counters serial command clears the counters on the serial links that are displayed using the show interfaces serial command. By placing the serial syntax after the show interfaces command, you are only displaying the serial interfaces on the router. The following is the output from using this command:

 Sean2514#  clear counters serial 0  Clear "show interface" counters on this interface [confirm] 03:54:37: %CLEAR-5-COUNTERS: Clear counter on interface Serial0 by console Sean2514# 

You will see the counters reset after using the command. Let's take a look:

 Sean2514#  show interfaces serial 0  1d03h: %CLEAR-5-COUNTERS: Clear counter on interface Serial0 by console Serial0 is up, line protocol is up   Hardware is HD64570   Internet address is 63.78.39.174/24   MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec, rely 255/255, load 1/255   Encapsulation FRAME-RELAY IETF, loopback not set, keepalive set (10 sec)   LMI enq sent  1, LMI stat recvd 1, LMI upd recvd 0, DTE LMI up   LMI enq recvd 0, LMI stat sent  0, LMI upd sent  0   LMI DLCI 1023  LMI type is CISCO  frame relay DTE   Broadcast queue 0/64, broadcasts sent/dropped 0/0, interface broadcasts 0   Last input 00:00:01, output 00:00:02, output hang never   Last clearing of "show interface" counters 00:00:02   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  0/1/256 (active/max active/max total)      Reserved Conversations 0/0 (allocated/max allocated)   5 minute input rate 0 bits/sec, 0 packets/sec   5 minute output rate 0 bits/sec, 0 packets/sec      0 packets input, 0 bytes, 0 no buffer      Received 0 broadcasts, 0 runts, 0 giants, 0 throttles      0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort      0 packets output, 0 bytes, 0 underruns      0 output errors, 0 collisions, 0 interface resets      0 output buffer failures, 0 output buffers swapped out      0 carrier transitions      DCD=up  DSR=up  DTR=up  RTS=up  CTS=up Sean2514# 

The debug serial interface Command

This debug command is used to provide real-time information on the line status, DTE status, and Serial interface sequencing information. Debugging of the serial interface also displays information on the HDLC and Frame Relay communication messages. Here is an example of output for HDLC messages:

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You need to remember that the debug commands can be used to help the Cisco Technical Assistance Center decipher problems with the router or serial interfaces. If you cannot resolve a problem displayed in the debug output, the Cisco TAC most likely has the resources to resolve it.


 Sean2514#  debug serial interface  Serial network interface debugging is on 1d03h: Serial0(out): StEnq, myseq 80, yourseen 79, DTE up 1d03h: Serial0(in):  Status, myseq 80 1d03h: Serial0(out): StEnq, myseq 81, yourseen 80, DTE up 1d03h: Serial0(in):  Status, myseq 81 1d03h: Serial0(out): StEnq, myseq 82, yourseen 81, DTE up 1d03h: Serial0(in):  Status, myseq 82 1d03h: Serial0(out): StEnq, myseq 83, yourseen 82, DTE up 

Table 10.1 defines the acronyms from the preceding output.

Table 10.1. Acronyms Displayed in the debug serial interface Command

Field

Description

StEnq

The LMI status inquiry sent from the LMI (router) to the local service provider's Frame Relay switch.

Status

The reply sent to the LMI from the Frame Relay switch.

mysec

The local keepalive number, also known as the sequence identifier.

yourseen

The keepalive sent by the opposite side of the serial connection.

DTE

The data-termination equipment status.

in/out

Specifies the direction the packets were sent through the interface. Outbound packets are keepalives sent by the local interface. Inbound packets are the keepalives sent from the opposite side of the serial link.

When the sequence numbers for an interface don't increment, the cause may be a timing or line issue. If two out of six consecutive keepalive packets fail to increment their sequence numbers , the line resets. Using the debug serial interface command, look at the debugging output for the HDLC protocol on a Cisco 2514 router:

 Sean2514#  debug serial interface  Serial network interface debugging is on 03:52:22: Serial0: HDLC myseq 1390, mineseen 1390*, yourseen 1404, line up 03:52:32: Serial0: HDLC myseq 1391, mineseen 1391*, yourseen 1405, line up 03:52:42: Serial0: HDLC myseq 1392, mineseen 1392*, yourseen 1406, line up 03:52:52: Serial0: HDLC myseq 1393, mineseen 1393*, yourseen 1407, line up 

In this situation shown above, the Layer 3 protocol considers the line protocol to be down, but the protocol operating at Layer 2 continues to send keepalive messages. As soon as the Layer 2 protocol receives three consecutive sequenced keepalive packets, the line protocol will be brought back up.

The show buffers Command

The show buffers command is used to show the router buffer pool statistics. Buffer pools are areas where the router stores received and sent data before it is processed , returned, or forwarded out an interface. The output is shown below:

 Sean2514#  show buffers  Buffer elements:      500 in free list (500 max allowed)      3621 hits, 0 misses, 0 created Public buffer pools:  Small buffers, 134 bytes (total 50, permanent 50):  44 in free list (20 min, 150 max allowed)      1226 hits, 0 misses, 0 trims, 0 created  0 failures (0 no memory)  Middle buffers, 600 bytes (total 25, permanent 25):      25 in free list (10 min, 150 max allowed)      22 hits, 0 misses, 0 trims, 0 created      0 failures (0 no memory) Big buffers, 1524 bytes (total 50, permanent 50):      50 in free list (5 min, 150 max allowed)      222 hits, 0 misses, 0 trims, 0 created      0 failures (0 no memory) VeryBig buffers, 4520 bytes (total 10, permanent 10):      10 in free list (0 min, 100 max allowed)      0 hits, 0 misses, 0 trims, 0 created      0 failures (0 no memory) Large buffers, 5024 bytes (total 0, permanent 0):      0 in free list (0 min, 10 max allowed)      0 hits, 0 misses, 0 trims, 0 created      0 failures (0 no memory) Huge buffers, 18024 bytes (total 0, permanent 0):      0 in free list (0 min, 4 max allowed)      0 hits, 0 misses, 0 trims, 0 created      0 failures (0 no memory) Interface buffer pools: Ethernet0 buffers, 1524 bytes (total 32, permanent 32):      8 in free list (0 min, 32 max allowed)      24 hits, 0 fallbacks      8 max cache size, 8 in cache Ethernet1 buffers, 1524 bytes (total 32, permanent 32):      8 in free list (0 min, 32 max allowed)      24 hits, 0 fallbacks      8 max cache size, 8 in cache Serial0 buffers, 1524 bytes (total 32, permanent 32):      7 in free list (0 min, 32 max allowed)      25 hits, 0 fallbacks      8 max cache size, 8 in cache Serial1 buffers, 1524 bytes (total 32, permanent 32):      7 in free list (0 min, 32 max allowed)      25 hits, 0 fallbacks      8 max cache size, 8 in cache Sean2514# 

The small buffers shown above in bold show all the frames received that are under 104 bytes. The failures indicate the number of frames that were discarded due to unavailable memory in the buffers.

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When analyzing the output of the show buffers command, remember that buffer hits are good; when the number of misses increases , you should concern yourself with the number of buffer misses.


The show controllers serial Command

Using the show controllers serial command, you can display the interface status, and whether a data terminal equipment (DTE) cable or a data communication equipment (DCE) cable is connected to an interface or is reversed . Rising hardware and data link errors may indicate a problem with the cable or a misconfigured interface.

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This is a particularly good command if you have a router in a rack and can't physically see that the serial cables are connected properly to the interface.


The following is the output you should see from the show controllers serial command (this output is the DCE end of the cable; the DTE end is connected to the serial port on a Cisco 1005):

 Sean2514#  show controller serial 0  HD unit 0, idb = 0xF7A98, driver structure at 0xFCF18 buffer size 1524  HD unit 0, V.35 DCE cable, clockrate 64000 cpb = 0x62, eda = 0x2918, cda = 0x292C RX ring with 16 entries at 0x622800 00 bd_ptr=0x2800 pak=0x0FE350 ds=0x6267C4 status=80 pak_size=22 01 bd_ptr=0x2814 pak=0x0FE150 ds=0x626108 status=80 pak_size=22 02 bd_ptr=0x2828 pak=0x0FDF50 ds=0x625A4C status=80 pak_size=22 03 bd_ptr=0x283C pak=0x0FFF50 ds=0x62C60C status=80 pak_size=22 04 bd_ptr=0x2850 pak=0x100150 ds=0x62CCC8 status=80 pak_size=22 05 bd_ptr=0x2864 pak=0x0FFD50 ds=0x62BF50 status=80 pak_size=22 06 bd_ptr=0x2878 pak=0x0FFB50 ds=0x62B894 status=80 pak_size=278 07 bd_ptr=0x288C pak=0x0FF950 ds=0x62B1D8 status=80 pak_size=22 08 bd_ptr=0x28A0 pak=0x0FF750 ds=0x62AB1C status=80 pak_size=22 09 bd_ptr=0x28B4 pak=0x0FF550 ds=0x62A460 status=80 pak_size=22 10 bd_ptr=0x28C8 pak=0x0FF350 ds=0x629DA4 status=80 pak_size=22 11 bd_ptr=0x28DC pak=0x0FF150 ds=0x6296E8 status=80 pak_size=22 12 bd_ptr=0x28F0 pak=0x0FEF50 ds=0x62902C status=80 pak_size=22 13 bd_ptr=0x2904 pak=0x0FED50 ds=0x628970 status=80 pak_size=278 14 bd_ptr=0x2918 pak=0x0FEB50 ds=0x6282B4 status=80 pak_size=22 15 bd_ptr=0x292C pak=0x0FE750 ds=0x62753C status=80 pak_size=22 16 bd_ptr=0x2940 pak=0x0FE550 ds=0x626E80 status=80 pak_size=278 cpb = 0x62, eda = 0x3014, cda = 0x3014 TX ring with 1 entries at 0x623000 00 bd_ptr=0x3000 pak=0x000000 ds=0x653E04 status=80 pak_size=284 01 bd_ptr=0x3014 pak=0x000000 ds=0x622EA4 status=80 pak_size=22 0 missed datagrams, 0 overruns 0 bad datagram encapsulations, 0 memory errors 0 transmitter underruns 0 residual bit errors Sean2514# 

The next code block is the output from the DTE end of the interface, connected to a Cisco 1005 router:

 Seans1005#  show controllers serial 0  QUICC Serial unit 0, idb at 0x22C2C20, driver data structure at 0x22C495C SCC Registers: General [GSMR]=0x2:0x00000030, Protocol-specific [PSMR]=0x0 Events [SCCE]=0x0000, Mask [SCCM]=0x001F, Status [SCCS]=0x0006 Transmit on Demand [TODR]=0x0, Data Sync [DSR]=0x7E7E Interrupt Registers: Config [CICR]=0x00368460, Pending [CIPR]=0x04004804 Mask   [CIMR]=0x48000012, In-srv  [CISR]=0x00000000 Command register [CR]=0x6C0 Port A [PADIR]=0x0008, [PAPAR]=0x5AC3        [PAODR]=0x0000, [PADAT]=0xD06A Port B [PBDIR]=0x020F1F, [PBPAR]=0x0000C0        [PBODR]=0x000000, [PBDAT]=0x000EC0 Port C [PCDIR]=0x038C, [PCPAR]=0x0001        [PCSO]=0x0C30,  [PCDAT]=0x0300, [PCINT]=0x0000 DTE V.35 serial cable attached. TX and RX clocks detected. SCC GENERAL PARAMETER RAM (at 0xFF00F00) Rx BD Base [RBASE]=0x560, Fn Code [RFCR]=0x18 Tx BD Base [TBASE]=0x5E0, Fn Code [TFCR]=0x18 Max Rx Buff Len [MRBLR]=1524 Rx State [RSTATE]=0x18008240, BD Ptr [RBPTR]=0x598 Tx State [TSTATE]=0x18000348, BD Ptr [TBPTR]=0x5E8 SCC HDLC PARAMETER RAM (at 0xFF00F38) 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]=1524 Rx Int Threshold [RFTHR]=0, Frame Cnt [RFCNT]=55929 User-defined Address 0000/0000/0000/0000 User-defined Address Mask 0x0000 buffer size 1524 RX ring with 16 entries at 0xFF00560, Buffer size 1524 Rxhead = 0xFF00598 (7), Rxp = 0x22C4994 (7) 00 pak=0x22C7B3C buf=0x2395FE0 status=9000 pak_size=0 01 pak=0x22C798C buf=0x2395928 status=9000 pak_size=0 02 pak=0x22C77DC buf=0x2395270 status=9000 pak_size=0 03 pak=0x22C762C buf=0x23A1548 status=9000 pak_size=0 04 pak=0x22C747C buf=0x23A0E90 status=9000 pak_size=0 05 pak=0x2309D88 buf=0x23B942C status=9000 pak_size=0 06 pak=0x230A7A8 buf=0x23BBC7C status=9000 pak_size=0 07 pak=0x230A0E8 buf=0x23BA19C status=9000 pak_size=0 08 pak=0x230A958 buf=0x23BC334 status=9000 pak_size=0 09 pak=0x230A448 buf=0x23BAF0C status=9000 pak_size=0 10 pak=0x230A5F8 buf=0x23BB5C4 status=9000 pak_size=0 11 pak=0x2309BD8 buf=0x23B8D74 status=9000 pak_size=0 12 pak=0x2309A28 buf=0x23B86BC status=9000 pak_size=0 13 pak=0x2309F38 buf=0x23B9AE4 status=9000 pak_size=0 14 pak=0x22C7E9C buf=0x2396D50 status=9000 pak_size=0 15 pak=0x22C7CEC buf=0x2396698 status=B000 pak_size=0 TX ring with 2 entries at 0xFF005E0, tx_count = 0 tx_head = 0xFF005E8 (1), head_txp = 0x22C49D8 (1) tx_tail = 0xFF005E8 (1), tail_txp = 0x22C49D8 (1) 00 pak=0x0000000 buf=0x0000000 status=0000 pak_size=0 01 pak=0x0000000 buf=0x0000000 status=2000 pak_size=0 QUICC SCC specific errors: 0 input aborts on receiving flag sequence 0 throttles, 0 enables 0 overruns 0 transmitter underruns 0 transmitter CTS losts Seans1005# 

The show controllers serial command output above provides information on the interface status, cable types, missed datagrams, overruns, bad encapsulated frames, memory errors, underruns, clock rate, and bit errors. In addition, it indicates the interface clock rate, as well as the cable type that is connected to the interface.

The show interface serial Command

The show interface serial command provides a wealth of information to help you troubleshoot serial line and serial interface related problems. The output from this command is shown here:

 Sean2514#  show interface serial 0  Serial0 is up, line protocol is up   Hardware is HD64570   Internet address is 207.212.78.107/24   MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec, rely 255/255, load 1/255   Encapsulation HDLC, loopback not set, keepalive set (10 sec)   Last input 00:00:08, output 00:00:06, 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  0/1/256 (active/max active/max total)      Reserved Conversations 0/0 (allocated/max allocated)   5 minute input rate 0 bits/sec, 0 packets/sec   5 minute output rate 0 bits/sec, 0 packets/sec      1609 packets input, 94022 bytes, 0 no buffer      Received 1609 broadcasts, 0 runts, 0 giants, 0 throttles      0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort      1600 packets output, 97280 bytes, 0 underruns      0 output errors, 0 collisions, 1 interface resets      0 output buffer failures, 0 output buffers swapped out      2 carrier transitions      DCD=up  DSR=up  DTR=up  RTS=up  CTS=up 
graphics/alert_icon.gif

For the exam you should know that the clear counters serial command can be used to clear the counters on only the serial interfaces.


Table 10.2 shows the description of the variables found in the preceding output.

Table 10.2. The Description of Variables from the show interface serial Command

Variable

Description

Serial0

Shows the port or VLAN number of the displayed interface, and the status indicates whether the interface is active or disabled. A status of "disabled" indicates that the router has received more than 5,000 errors in 10 seconds, which is the default keepalive interval.

Line protocol

An indicator of whether the line protocol believes the interface is usable, based on the keepalives it has received, or whether the interface has been manually shut down.

Internet address

The IP address of the interface.

MTU

Displays the maximum transmission unit (MTU) size for the interface.

BW

Displays the bandwidth the interface is configured to use. You can modify this parameter by using the bandwidth command in Interface Configuration mode.

DLY

Displays the interface delay in microseconds.

Rely

Displays the interface in a fraction of 255. This means that if the interface is running at 100 percent, the displayed value would be 255 out of 255.

Load

Displays the interface load in a fraction of 255. This means that if the interface is running a 100 percent load, the displayed value would be 255 out of 255.

Encapsulation

Displays the encapsulation method used by the interface.

ARP Type

Displays the frames encapsulation type, such as ARPA, SNAP, Novell- Ether , or SAP.

Loopback

Indicates whether the interface is in loopback mode.

Keepalives

Indicates whether keepalives are being sent.

Last Input/Output

Indicates how much time has occurred since the last packet was received by the interface.

Last Clearing

Indicates the amount of time since the counters have been reset.

Input/Output Queues

Displays the maximum size of the queue, as well as the number of packets dropped due to a full queue.

Input/Output Rates

Displays the average number of bits and packets transmitted in the last 300 seconds (5 minutes).

Packets Input

Displays the number of successfully received packets to the interface since the last interface counter reset.

No Buffer

Displays the number of received packets discarded by a full buffer.

Broadcasts

Displays the number of multicast and broadcast packets received.

Runts

Displays the number of packets discarded because they were smaller than the MTU size allowed on the physical media.

Giants

Displays the number of packets discarded because they exceeded the maximum MTU size of the physical media.

CRC

Displays the number of CRC errors received on an interface.

Frame

Displays the number of packets received with CRC errors.

Overrun

Displays the number of times the receiver was unable to receive data into its buffers because the data input rate exceeded the rate which the receiving buffers and interface could handle.

Underruns

Displays the number of instances in which the transmitter has run faster than the router.

Output Errors

Displays the total number of transmitted errors on the interface.

Collisions

Displays the number of collisions that took place with transmitted media on an Ethernet interface.

Interface Resets

Displays a count of the number of times an interface has been recycled.

Ignored

Displays the number of packets ignored by the interface due to exceeded buffer demands.

Packets Output

Displays the total number of packets transmitted on the interface.

Input Errors

Displays the total number of errors, which includes the runts, giants, CRCs, discarded frames, overruns, and ignored errors.

graphics/alert_icon.gif

For the exam, you can expect that several questions will involve a knowledge of the output from the show interfaces serial command. The variables and their descriptions are located in Table 10.2.




CCNP CIT Exam Cram 2 (642-831)
CCNP CIT Exam Cram 2 (Exam Cram 642-831)
ISBN: 0789730219
EAN: 2147483647
Year: 2003
Pages: 213
Authors: Sean Odom

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