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Chapter 9      Troubleshooting
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...

 
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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...

 
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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...

 
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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...

 
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Chapter 9      Troubleshooting
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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...

 
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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...

 
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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,...

 
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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...

 
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Chapter 9      Troubleshooting
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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)...

 
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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...