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Troubleshooting Serial Line Problems
Performing Ping Tests
Use the following steps to perform ping tests:
Step 1Put the CSU or DSU into local loopback mode.
Step 2Use the ping privileged EXEC command to send different data patterns and 
packet sizes. 
Performing Loopback Tests
These loopback tests do not apply to Frame Relay or X.25 networks.
Local Loopback Tests
Follow the steps below to perform local loopback tests:
Step 1Place the CSU/DSU in local loop mode (refer to your CSU/DSU 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 2Enter the show interfaces serial 0 privileged EXEC command to determine if the 
line status changes from “line protocol is down” to “line protocol is up (looped),” 
or if it remains down.
If the line protocol comes up when the CSU or DSU is in local loopback mode, a 
problem could be 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.
If the problem appears to be local, enter the debug serial interface privileged 
EXEC command and go on to the next step.
Step 3Take 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. 
						

							 
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Step 4Place the CSU/DSU in local loop mode again. 
This action 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. 
Step 5Check the local router and CSU/DSU hardware, and any attached cables. 
Make certain 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 the cables are 
attached to the proper ports. Swap faulty equipment as necessary.
Remote Loopback Tests
Follow the steps below to perform remote loopback tests:
Step 1Put the remote CSU or DSU into remote loopback mode (refer to the your 
CSU/DSU documentation).
Step 2Enter the show interfaces serial 0 privileged EXEC command to determine if the 
line protocol remains up with the status line indicating “Serial x is up, line 
protocol is up (looped),” or if it goes down with the status line indicating “line 
protocol is down.”
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.
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. The 
CSU/DSU must be configured to use the same framing and coding schemes used 
by the leased-line or other carrier service (for example, ESF and B8ZS). 
						

							 
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Synchronous Leased Line Problems
Follow the steps below to troubleshoot problems with your synchronous leased 
line:
Step 1From privileged EXEC command mode, enter the show interfaces serial 0 
command.
If you see the line 
Serial0 is up, line protocol is up, the serial line is 
functioning properly. You do not need to take further action.
Step 2If you see one of the following messages, refer to Ta b l e 9 - 9:
Serial 0 is down, line protocol is down.
Serial 0 is up, line protocol is down.
Serial 0 is up, line protocol is up (looped).
Serial 0 is administratively down, line protocol is up. 
						

							 
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Table 9-9 Leased Line Problems
Line State Problem Solution
Serial 0 is down; 
line protocol is 
down.The router is not sensing a carrier detect 
(CD) signal as a result of one of the 
following:
Faulty or incorrect cabling of the 
router.
Local router hardware failure.
Local CSU/DSU hardware failure.
WAN service provider problem, 
such as the line is down or not 
connected to the CSU/DSU.Following are some steps you can 
take to isolate the problem:
Refer to the Cisco 805 Router 
Hardware Installation Guide to 
confirm that you are using the 
correct serial cable to connect 
the CSU/DSU and that you 
connected the CSU/DSU 
correctly.
Connect the leased line to 
another port, if possible. If the 
connection comes up, there is a 
hardware failure. Contact your 
Cisco reseller.
Check the LEDs on the 
CSU/DSU for CD activity. 
Contact your WAN service 
provider. 
						

							 
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Serial 0 is up; 
line protocol is 
down.Possible causes for this line state are:
Router hardware failure.
Local or remote CSU/DSU hardware 
failure.
Local or remote router 
misconfigured.
The serial clock transmit external is 
not set on the CSU/DSU.
The remote router is not sending 
keepalive packets.
Problem with the leased line.Following are some steps you can 
take to isolate the problem:
Refer to the Cisco 805 Router 
Hardware Installation Guide to 
confirm that you are using the 
correct serial cable to connect 
the CSU/DSU and that you 
connected the CSU/DSU 
correctly.
Connect the leased line to 
another port, if possible. If the 
connection comes up, there is a 
hardware failure. Contact your 
Cisco reseller.
Check the LEDs on the 
CSU/DSU for CD activity. 
Perform CSU/DSU loopback 
tests. During local loopback, 
enter the show interfaces serial 
0 command. If the line protocol 
is shown as up, there might be a 
problem with the WAN service 
provider, or the remote router is 
down.
Contact your WAN service 
provider.
Table 9-9 Leased Line Problems (continued)
Line State Problem Solution 
						

							 
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Serial 0 is up; 
line protocol is up 
(looped).The possible cause is a loop in the circuit. 
The sequence number in the keepalive 
packet changes to a random number 
when a loop is first detected. If the same 
random number is returned over the line, 
a loop exists.Following are some steps you can 
take to isolate the problem:
Use the write terminal 
privileged EXEC command to 
display any instances of the 
loopback command. If the router 
has been configured with the 
loopback command, enter the 
no loopback command to 
remove the loop.
Check to see whether the 
CSU/DSU is configured in 
manual loopback mode. If it is, 
disable manual loopback.
Reset the CSU/DSU.
If you are unable to isolate the 
problem, contact your WAN 
service provider for help with 
troubleshooting.
Table 9-9 Leased Line Problems (continued)
Line State Problem Solution 
						

							 
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Chapter 9      Troubleshooting
Troubleshooting Serial Line Problems
Asynchronous Dial-Up Problems
This section describes how to use the show line 1 command to troubleshoot 
problems with the connection between your modem and router. It also describes 
the following symptoms, problems, and solutions:
Troubleshooting Problems with Modem and Router Connection, page 9-26
No Connectivity Between Modem and Router, page 9-28
Modem Does Not Dial, page 9-29 Serial 0 is 
administratively 
down; line protocol 
is up.The possible causes for this state are:
The serial interface has been 
disabled with the shutdown 
command.
Different interfaces on the router are 
using the same IP address.Following are some steps you can 
take to isolate the problem:
Use the show configuration 
privileged EXEC command to 
display the serial port 
configuration. If “shutdown” is 
displayed after “interface 
Serial0,” use the no shutdown 
command in serial interface 
configuration mode to enable the 
interface.
Use the show interface 
privileged EXEC command to 
display the IP addresses for all 
router interfaces. Take the 
appropriate action to assign a 
unique IP address to each router 
interface. (If you set up your 
network per the sample networks 
in the this guide, refer to that 
particular sample network for 
information on how to assign a 
unique IP address to the router 
interfaces. 
Table 9-9 Leased Line Problems (continued)
Line State Problem Solution 
						

							 
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Modem Does Not Answer, page 9-30
Modem Hangs Up Shortly After Connecting, page 9-32
Dial-Up Client Receives No EXEC Prompt, page 9-34
Dial-Up Session Sees Garbage, page 9-35
Dial-Up Session Ends Up in Existing Session, page 9-36
Modem Cannot Send or Receive Data, page 9-37
Modem Cannot Send or Receive IP Data, page 9-39
Modem Does Not Disconnect Properly, page 9-41
Link Deactivates Too Quickly, page 9-42
Link Does Not Deactivate or Stays Activated Too Long, page 9-42
Poor Dial-Up Connection Performance, page 9-43
Troubleshooting Problems with Modem and Router Connection
Follow the steps below to troubleshoot problems with the connection between 
your modem and router:
Step 1In privileged EXEC command mode, enter the show line 1 command.
Check the Modem state field in the output. If the modem state is Idle and CTS 
noDSR DTR RTS, the connection between your modem and router is functioning 
properly. 
Step 2If you see one of the following modem states, see Ta b l e 9 - 1 0:
Ready  –
Ready not CTS noDSR DTR RTS
Ready CTS DSR DTR RTS
Ready CTS* DSR* DTR RTS 
						

							 
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Table 9-10 Problems with Modem and Router Connection
Modem State Problem
Ready –
Modem control is not configured on the router. Enter the modem 
inout command in serial interface configuration mode.
A session exists on the line. Enter the show users privileged EXEC 
command and the clear line 0 privileged EXEC command to stop the 
session if desired.
Data set ready (DSR) is high. There are two possible reasons for this:
–Cabling problems—If your modem connector uses DB-25 pin 6 
and has no pin 8, you must move the pin from 6 to 8 or get the 
appropriate connector. 
–Modem configured for data carrier detect (DCD) always 
high—The modem should be reconfigured to have DCD high only 
on carrier detect (CD), which is usually done with the &C1 
modem command. Check your modem documentation for the 
exact syntax for your modem.
If your software does not support modem control, you must configure 
the router line to which the modem is connected with the no exec 
command in asynchronous line configuration mode. Clear the line 
with the clear line privileged EXEC command, initiate a reverse 
Telnet session with the modem, and reconfigure the modem so that 
DCD is high only on CD. End the Telnet session by entering 
disconnect and reconfigure the router line with the EXEC command 
in asynchronous line configuration mode.
Ready noCTS noDSR 
DTR RTS
The modem is turned off.
The modem is not properly connected to the router. Refer to the 
Cisco 805 Router Hardware Installation Guide for information on 
how to select the serial cable and how to connect the modem.
The modem is not configured for hardware flow control. Disable 
hardware flow control on the router by entering the no flowcontrol 
hardware command in asynchronous line configuration mode. 
Enable hardware flow control on the modem via a reverse Telnet 
session. (Consult your modem documentation.) Reenable hardware 
flow control on the router by entering the flowcontrol hardware 
command in asynchronous line configuration mode. 
						

							 
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No Connectivity Between Modem and Router
The connection between a modem and a Cisco router does not work. Attempts to 
initiate a reverse Telnet session to the modem have no result, or you receive a 
“Connection Refused by Foreign Host” message.
Ta b l e 9 - 1 1 outlines the problems that might cause this connectivity failure and 
describes possible solutions. Ready CTS DSR DTR 
RTS
Incorrect cabling. Refer to the Cisco 805 Router Hardware 
Installation Guide for information on how to select the serial cable.
The modem is configured for DCD always high. Reconfigure the 
modem so that DCD is only high on CD, which is usually done with 
the &C1 modem command. Check your modem documentation for 
the exact syntax for your modem.
Configure the router line to which the modem is connected by entering the 
no exec command in asynchronous line configuration mode. Clear the line 
with the clear line privileged EXEC command, initiate a reverse Telnet 
session with the modem, and reconfigure the modem so that DCD is high 
only on CD. End the Telnet session by entering disconnect. Reconfigure 
the router line with the exec command in asynchronous line configuration 
mode.
NoteReady CTS* 
DSR* DTR RTS1If this string appears in the Modem state field, modem control is probably 
not enabled on the router. Enter the modem inout command in 
asynchronous line configuration mode to enable modem control on the 
line.
1. An asterisk (*) next to a signal indicates one of two things: Either the signal has changed within the last few seconds, or the 
signal is not being used by the modem control method selected.
Table 9-10 Problems with Modem and Router Connection (continued)
Modem State Problem