Network Address Translation
Wireless Network Planning Around Interference
Types of Network Address Translation

Troubleshooting Serial Connections

To effectively identify and troubleshoot any problem, including those with your serial connections, you must have the key information about the devices or services you are troubleshooting. There are some show and debug commands you can use to identify configuration or flow issues with serial connections.

Showing

The show command displays the configuration and status of your serial ports. In the following code, the items that stand out the most are the line protocols that are reported as down, meaning that the physical connection to the device at the other end of the connection is not established, as well as the encapsulation protocol that is in use, such as HDLC or PPP.

Router1# show interfaces serial 0
Serial0 is up, line protocol is down
  Hardware is HD64570
  Internet address is 24.0.0.1/8
  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 never, output 00:00:08, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); 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)
     Available Bandwidth 1158 kilobits/sec
  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
     21 packets output, 714 bytes, 0 underruns
     0 output errors, 0 collisions, 12 interface resets
     0 output buffer failures, 0 output buffers swapped out

After correcting the preceding problem — in this case, the clock speed was not set on the DCE device — the line protocol is reported as being up:

Router1#show interfaces serial 0
Serial0 is up, line protocol is up
  Hardware is HD64570
  Internet address is 24.0.0.1/8
  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 00:00:09, output 00:00:08, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); 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)
     Available Bandwidth 1158 kilobits/sec
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     8 packets input, 1208 bytes, 0 no buffer
     Received 8 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     91 packets output, 3282 bytes, 0 underruns
     0 output errors, 0 collisions, 43 interface resets
     0 output buffer failures, 0 output buffers swapped out
     85 carrier transitions
     DCD=up  DSR=up  DTR=up  RTS=up  CTS=up

Debugging

After you know how to view current configuration information, take a look at the active connection and data going through the connection by using the debug command. You can choose from the following debug options. which focus on the interface option.

Router1#debug serial ?
  interface  Serial interface events
  m32_dma    Serial M32 DMA
  mueslix    Serial Mueslix
  packet     Serial network interface packets
  revive     Serial Revive

When you see the show command named Showing in the preceding section, you start with an issue with clock speed not being properly set on the DCE side of the connection. To see and diagnose a problem, the problem was re-created the problem so that you can see what it would look like when troubleshooting with the debug command.

When you have an issue with your connection, you can take a look at the information the debug command gives you about the serial interface. After the code example, some information that you should be able to deduce is provided.

Router1#debug serial interface
Serial network interface debugging is on
Router1#
00:06:33: Serial0: HDLC myseq 32, mineseen 0, yourseen 0, line down
00:06:34: Serial0: attempting to restart
00:06:34: HD(0): Deasserting DSR, CTS and DCD
00:06:34: HD(0): Reset from 0x305CE4A
00:06:34: HD(0): Asserting DSR
00:06:34: HD(0): Asserting DCD and CTS
00:06:34: HD(0): Deasserting LTST
00:06:34: HD(0): Asserting DTR and RTS
00:06:43: Serial0: HDLC myseq 33, mineseen 0, yourseen 0, line down
00:06:45: HD(0): got an interrupt state = 0x8057
00:06:45: HD(0): New serial state = 0x0055
00:06:45: HD(0): DTR is up.
00:06:45: HD(1): New serial state = 0x0600
00:06:45: HD(1): Cable is unplugged.
00:06:45: HD(0): got an interrupt state = 0x805F
00:06:45: HD(0): New serial state = 0x005F
00:06:45: HD(0): DTR is up.
00:06:45: HD(1): New serial state = 0x0600
00:06:45: HD(1): Cable is unplugged.

Here are some key items to take note of in this output:

  • The connection is using HDLC. The HDLC connection is down.

  • There is an issue with communications, as shown by the asserting, deasserting, and reset commands. These are typically never good messages to see. Specifically, there is a problem in confirming serial communication commands, such as Ready to Send (RTS) and Clear to Send (CTS).

  • There is an issue with the HDLC communication, and after several attempts, it gives up, identified by the Cable is unplugged statement.

Another issue that shows up in the output is related to the lines that include the words mineseen and yourseen. They refer to the number of data control packets that have been sent over the connection, which are used only to monitor the connection.

When the router sends data over an HDLC connection, it tracks a sequential number to ensure that all the data gets through. The number on Router1 is what Router1 calls mineseen, whereas Router1 call the sequence number reported by the other router yourseen.

Although HDLC is saying that the cable may be unplugged, you already know that the problem lies in the clock speed, which to Router1 looks the same because it cannot start the connection properly.

With the clock speed correctly set, the debug data looks like the following example. Notice the mineseen and yourseen data and that the line is up. The difference in the numbers occurs because Router2 has been sending data control packets that this router has never seen, because the interface was brought up later.

Router1#debug serial interface
Serial network interface debugging is on
Router1#
00:22:43: Serial0: HDLC myseq 10, mineseen 10*, yourseen 129, line up
00:22:53: Serial0: HDLC myseq 11, mineseen 11*, yourseen 130, line up
00:23:03: Serial0: HDLC myseq 12, mineseen 12*, yourseen 131, line up
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