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Chapter 9      Troubleshooting
Troubleshooting Serial Line Problems
Modem Cannot Send or Receive IP Data
After a dial-up connection is established, a modem cannot send or receive IP data.
Ta b l e 9 - 1 9 outlines causes of this problem and describes possible solutions. Hardware flow control not 
configured on local or 
remote modem or router
Enter the show line 0 privileged EXEC command, and look for the 
following in the Capabilities field:
Capabilities: Hardware Flowcontrol In, Hardware Flowcontrol 
Out...
If there is no mention of hardware flow control in this field, hardware 
flow control is not enabled on the line. 
Configure hardware flow control on the line using the flowcontrol 
hardware command in asynchronous line configuration mode. If for 
some reason you cannot use flow control, limit the line speed to 
9600 bps. Faster speeds are likely to result in lost data.
After enabling hardware flow control on the router line, initiate a 
reverse Telnet session to the modem via that line. 
Use a modem command string that includes the RTS/CTS flow 
command for your modem. This command ensures that the modem is 
using the same method of flow control (that is, hardware flow control) 
as the Cisco router is using. See your modem documentation for the 
exact configuration command syntax. 
Problem with dialing 
modemMake sure that the dialing modem is operational and is securely 
connected to the correct port. Check whether another modem works 
when connected to the same port.
Table 9-18 Modem Cannot Send or Receive Data (continued)
Problem Solution 
						

							 
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Table 9-19 Modem Cannot Send or Receive IP Data
Problem Solution
IP routing not enabled on 
local or remote routerMake sure that IP routing is enabled on the local and remote routers.
No default gateway 
specified on PC
Enter the show slip privileged EXEC command. Make sure that the 
specified IP address is the same as the default gateway specification 
on the PC.
Check the specified default gateway address on the PC. If the IP 
address is not correct, specify the correct address. For instructions on 
verifying and changing the default gateway address on the 
workstation, refer to the vendor documentation.
Hardware flow control not 
configured on local or 
remote modem or router
Enter the show line 0 privileged EXEC command, and look for the 
following in the Capabilities field:
Capabilities: Hardware Flowcontrol In, Hardware Flowcontrol 
Out...
If there is no mention of hardware flow control in this field, hardware 
flow control is not enabled on the line. 
Configure hardware flow control on the line using the flowcontrol 
hardware command in asynchronous line configuration mode. If for 
some reason you cannot use flow control, limit the line speed to 
9600 bps. Faster speeds are likely to result in lost data.
After enabling hardware flow control on the router line, initiate a 
reverse Telnet session to the modem via that line. 
Use a modem command string that includes the RT S/CTS flow 
command for your modem. This command ensures that the modem is 
using the same method of flow control (that is, hardware flow 
control) as the Cisco router is using. See your modem documentation 
for the exact configuration command syntax.  
						

							 
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Modem Does Not Disconnect Properly
The modem does not disconnect properly. Connections to the modem do not 
terminate when the quit command is entered.
Ta b l e 9 - 2 0 outlines causes of this problem and describes possible solutions. Static routes not 
configured
Use the show ip route privileged EXEC command to check whether 
there is a static route to the remote network in the routing table.
If there is no static route to the remote network, configure one, using 
the ip route command. The static route should point to the remote 
network.
Domain Name System 
(DNS) server not specified 
on router or workstation
Check whether the workstation and router both have DNS 
information specified. On the router, use the show running-config 
privileged EXEC command to see if DNS is configured. For 
information on verifying the workstation configuration, refer to the 
vendor documentation.
If the router and workstation are not configured to use DNS, use the 
ip domain-lookup, ip domain-name, and ip name-server 
commands to configure the router.
Configure a DNS server address in the TCP/IP software on the PC. 
For more information, refer to the vendor documentation.
Table 9-19 Modem Cannot Send or Receive IP Data (continued)
Problem Solution
Table 9-20 Modem Does Not Disconnect Properly
Problem Solution
Modem is not sensing 
DTR.Enter the hangup DTR modem command string. This command tells 
the modem to drop carrier when the DTR signal is no longer being 
received. For the exact syntax of this command, see the your modem 
documentation. 
						

							 
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Link Deactivates Too Quickly
After a dial-up connection is established, the link deactivates too quickly.
Ta b l e 9 - 2 1 outlines causes of this problem and describes possible solutions.
Link Does Not Deactivate or Stays Activated Too Long
After a dial-up connection is established, the link activates indefinitely or stays 
activated too long in an idle state.
Ta b l e 9 - 2 2 outlines causes of this problem and describes possible solutions. Modem control is not 
enabled on the router.
Use the show line 1 privileged EXEC command on the router. The 
output should show 
inout or RIisCD in the Modem column, which 
indicates that modem control is enabled on the line of the router. 
If necessary, configure modem control by using the modem inout 
command in asynchronous line configuration mode. 
Table 9-20 Modem Does Not Disconnect Properly (continued)
Problem Solution
Table 9-21 Link Deactivates Too Soon
Problem Solution
Dialer timeout is too short
Use the show running-config privileged EXEC command to view 
the router configuration. Check the value configured with the dialer 
idle-timeout command.
Increase the timeout value, using the dialer idle-timeout seconds 
command. The default is 120 seconds.
Dialer lists are too 
restrictive
Use the show running-config privileged EXEC command to view 
the router configuration. Check the access lists, if any, referenced by 
dialer list commands.
Make sure that the access lists describe all the traffic that should keep 
the link active. Reconfigure the access lists to include additional 
traffic if necessary. 
						

							 
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Poor Dial-Up Connection Performance
After a dial-up connection is established, performance over the link is slow or 
unreliable, often as a result of a high rate of data loss.
Ta b l e 9 - 2 3 outlines causes of this problem and describes possible solutions.
Table 9-22 Link Does Not Deactivate or Stays Activated Too Long
Problem Solution
Dialer lists not restrictive 
enough
Use the show running-config privileged EXEC command to view 
the router configuration. Check the access lists, if any, referenced by 
dialer list commands.
Make sure that the access lists do not describe traffic that should not 
keep the link active. Reconfigure the access lists if necessary.
Modems misconfigured Make sure that the local and remote modems are properly configured. In 
particular, both modems should be configured to disconnect on loss of 
DTR (Hangup DTR). For the exact syntax of this command, see your 
modem documentation. 
						

							 
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Table 9-23 Poor Dial-Up Connection Performance
Problem Solution
Error correction is not 
configured on the modem.Make certain the modem is configured for error correction. For the exact 
syntax of the command, see your modem documentation.
Flow control is not 
enabled, is enabled only on 
one device (either DTE or 
DCE), or is misconfigured.
Enter the show line 0 privileged EXEC command, and look for the 
following in the Capabilities field:
Capabilities: Hardware Flowcontrol In, Hardware Flowcontrol 
Out...
If there is no mention of hardware flow control in this field, hardware 
flow control is not enabled on the line. 
Configure hardware flow control on the line using the flowcontrol 
hardware command in asynchronous line configuration mode. If for 
some reason you cannot use flow control, limit the line speed to 
9600 bps. Faster speeds are likely to result in lost data.
After enabling hardware flow control on the router line, initiate a 
reverse Telnet session to the modem via that line. 
Use a modem command string that includes the RT S/CTS flow 
command for your modem. This command ensures that the modem is 
using the same method of flow control (that is, hardware flow 
control) as the Cisco router. See your modem documentation for the 
exact configuration command syntax.
Congestion or line noise.
If the network is congested, dial-up connections can freeze for a few 
seconds. The only solution is to reduce congestion on the network by 
increasing bandwidth or redesigning the network.
Line noise can also freeze up a dial-up connection. For information 
on how to account for line noise for your modem, refer to the vendor 
documentation. 
						

							 
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Frame Relay Problems
This section describes how to troubleshoot the following Frame Relay symptoms:
Frame Relay Link Is Down, page 9-45
Cannot Ping Remote Router, page 9-48
Cannot Ping End to End, page 9-51
Frame Relay Link Is Down
Connections over a Frame Relay link fail. Ta b l e 9 - 2 4 outlines causes of this 
problem and describes possible solutions.
Table 9-24 Frame Relay Link Is Down
Problem Solution
Cabling, hardware, or carrier 
problemPerform these steps on the local and remote routers.
Use the show interfaces serial 0 command to see if the interface 
and line protocol are up.
If the interface and line protocol are down, 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. Make sure that cables are 
securely attached.
If the cable is correct, try moving it to a different port. If that port 
works, then the first port is defective. Replace the router.
If the cable does not work on the second port, replace the cable. If 
it still does not work, there might be a problem with the DCE. 
Contact your carrier about the problem. 
						

							 
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Local management interface 
(LMI) type mismatchUse the show interfaces serial 0 command to check the state of 
the interface.
If the output shows the interface is up but the line protocol is down, 
enter the show frame-relay lmi privileged EXEC command to see 
which LMI type is configured on the Frame Relay interface.
Make sure that the LMI type is the same for all devices in the path 
from source to destination. Enter the frame-relay lmi-type {ansi 
| cisco | q933a} command in serial interface configuration mode to 
change the LMI type on the router.
Keepalives not being sent
Enter the show interfaces serial 0 command to find out if 
keepalives are configured. If you see a line that says “keepalives 
not set,” keepalives are not configured.
Use the keepalive seconds command in serial interface 
configuration mode to configure keepalives. The default value for 
this command is 10 seconds.
Encapsulation mismatch
When connecting Cisco devices with non-Cisco devices, you must 
use Internet Engineering Task Force (IETF) encapsulation on both 
devices. Check the encapsulation type on the Cisco device by 
using the show frame-relay map privileged EXEC command.
If the Cisco device is not using IETF encapsulation, use the 
encapsulation frame-relay ietf command in serial interface 
configuration mode to configure IETF encapsulation on the 
Cisco Frame Relay interface. For information on viewing or 
changing the configuration of the non-Cisco device, refer to the 
vendor documentation.
Table 9-24 Frame Relay Link Is Down (continued)
Problem Solution 
						

							 
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Cannot Ping Remote Router
Attempts to ping the remote router across a Frame Relay connection fail. 
Ta b l e 9 - 2 5 outlines causes of this problem and describes possible solutions. Data-link connection 
identifier (DLCI) inactive or 
deleted
Enter the show frame-relay pvc privileged EXEC command to 
view the status of the interface PVC.
If the output shows that the PVC is inactive or deleted, there is a 
problem along the path to the remote router. Check the remote 
router or contact your carrier to check the status of the PVC.
DLCI assigned to wrong 
subinterface
Use the show frame-relay pvc privileged EXEC command to 
check the assigned DLCIs. Make sure that the correct DLCIs are 
assigned to the correct subinterface.
If the DLCIs appear to be correct, shut down the main interface by 
entering the shutdown command in serial interface configuration 
mode, then bring the interface back up entering the no shutdown 
command.
Table 9-24 Frame Relay Link Is Down (continued)
Problem Solution
Table 9-25 Cannot Ping Remote Router
Problem Solution
Encapsulation mismatch
When connecting Cisco devices to non-Cisco devices, you must 
use IETF encapsulation on both devices. Check the encapsulation 
type on the Cisco device by using the show frame-relay map 
privileged EXEC command.
If the Cisco device is not using IETF encapsulation, use the 
encapsulation frame-relay ietf command in serial interface 
configuration mode to configure IETF encapsulation on the 
Cisco Frame Relay interface. For information on viewing or 
changing the configuration of the non-Cisco device, refer to the 
vendor documentation. 
						

							 
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DLCI inactive or deletedEnter the show frame-relay pvc privileged EXEC command to 
view the status of the interface PVC.
If the output shows that the PVC is inactive or deleted, there is a 
problem along the path to the remote router. Check the remote 
router, or contact your carrier to check the status of the PVC.
DLCI assigned to wrong 
subinterface
Use the show frame-relay pvc privileged EXEC command to 
check the assigned DLCIs. Make sure that the correct DLCIs are 
assigned to the correct subinterface.
If the DLCIs appear to be correct, shut down the main interface by 
entering the shutdown command in serial interface configuration 
mode. Then bring the interface back up by entering the no 
shutdown command.
Misconfigured access list
Enter the show access-list privileged EXEC command to see 
whether there are access lists configured on the router.
If access lists are configured, test connectivity by disabling access 
lists by entering the no access-group command in global 
configuration mode. Check to see whether connectivity is 
restored.
If connections work, reenable access lists one at a time, checking 
connections after enabling each access list.
If enabling an access list blocks connections, make sure that the 
access list does not deny necessary traffic. Make sure to configure 
explicit permit statements for any traffic you want to pass.
Continue testing access lists until all access lists are restored and 
connections still work.
Table 9-25 Cannot Ping Remote Router (continued)
Problem Solution