Several clear , show , and debug commands can be used to troubleshoot serial links. This section covers the following commands:
The clear counters serial CommandThe 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 CommandThis 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:
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
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 CommandThe 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.
The show controllers serial CommandUsing 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.
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 CommandThe 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
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
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