Troubleshooting ISDN Connections - N

Troubleshooting ISDN Connections - N
C H A P TER
15
Troubleshooting ISDN Connections
This chapter presents troubleshooting information for Integrated Services Digital Network (ISDN)
connectivity and performance problems. The sections in this chapter describe specific ISDN
symptoms, the problems that are likely to cause each symptom, and the solutions to those problems.
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ISDN: Router Does Not Dial
ISDN: Dial Does Not Go Through (BRI)
ISDN: Dial Does Not Go Through (PRI)
ISDN: No Communication with Remote Router
ISDN: No Communication End-to-End
ISDN: Second B Channel Does Not Come Up
ISDN: Second B Channel Comes Up Too Late
ISDN: Second B Channel Comes Up Too Early
ISDN: Slow Performance
ISDN: Line Disconnects Too Slowly
ISDN: Line Disconnects Too Quickly
Troubleshooting ISDN Connections 15-1
ISDN: Router Does Not Dial
ISDN: Router Does Not Dial
Symptom: Router configured for ISDN does not dial.
Table 15-1 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-1
ISDN: Router Does Not Dial
Possible Problem
Solution
Interface down
Step 1
Enter the show interfaces EXEC command to check the status of the ISDN interface.
Step 2
If the output of the show interfaces command indicates that the interface is
administratively down, bring the interface back up using the no shutdown interface
configuration command.
Step 3
If the interface or line protocol is down, check all cabling and connections.
Troubleshoot the hardware and the media. For more information, refer to the
“Troubleshooting Hardware and Booting Problems” chapter and the “Troubleshooting
Serial Line Problems” chapter.
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check to see if there are dialer map interface configuration commands
configured for the protocols you are using.
Step 2
If there is not a dialer map configured for the protocol you are using, create a dialer
map for each protocol.
Missing or misconfigured dialer map
commands
For example, if you want to configure a dialer map for IP, enter dialer map commands
similar to the following on the interface:
dialer map ip 172.16.20.2 name C4000 speed 56 broadcast 14155551234
dialer map ip 172.16.20.2 name C4000 speed 56 broadcast 14155556789
No dialer group configured
Missing or misconfigured dialer lists
Step 3
If there are already dialer map commands present, make sure that the next hop
address is in the same subnet as the local interface address.
Step 4
If you want broadcast traffic to trigger the dialer, make sure that the broadcast
keyword is specified in your dialer map statements.
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check to see if there are dialer-group interface configuration command
entries present for the interface.
Step 2
If the local interface does not belong to a dialer group, configure the interface as part
of a dialer group using the dialer-group group-number interface configuration
command. This command associates an interface with a dialer group.
Step 3
Make sure that the group-number is the same number used in the associated dialer-list
global configuration commands.
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check to see if there are dialer-list interface command entries present
for the interface.
Step 2
If there are no dialer lists configured, enter the dialer-list protocol or the dialer-list
list global configuration command to associate a dialer group with an access list.
In the following example, dialer group 1 is associated with access list 101:
dialer-list 1 list 101
Step 3
15-2 Internetwork Troubleshooting Guide
Make sure that dialer-list commands reference existing dialer groups and existing
access lists, or create the appropriate dialer groups or access lists before attempting to
dial.
ISDN: Router Does Not Dial
Possible Problem
Solution
Missing or misconfigured access lists
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check to see if the access list numbers specified in dialer-list
commands refer to existing access-list command entries.
Step 2
If the referenced access list is not defined, dialing will not occur. Configure access lists
that define interesting traffic and make sure that the lists are referenced correctly by
dialer-list commands.
In the following example, IGRP routing updates are classified as uninteresting (they
will not cause the router to dial), while all other IP packets are classified as interesting:
access-list 101 deny igrp 0.0.0.0 255.255.255.255 255.255.255.255
0.0.0.0
access-list 101 permit ip 0.0.0.0 255.255.255.255 0.0.0.0
255.255.255.255
Step 3
Missing pri-group command
If there are access lists already present and they are referenced correctly by dialer-list
commands, make sure that the traffic you want to trigger the dialer is defined as
interesting by the access list.
On Cisco 7000 series routers, use the pri-group controller configuration command to specify
ISDN PRI1 on a channelized T1 card.
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check to see if there is a pri-group command entry.
Step 2
If the command is not present, configure the controller with the pri-group command.
Following is an example configuration for a Cisco 7000 series router with a
channelized T1 card:
controller t1 0
framing esf
line code b8zs
pri-group timeslots 1-24
1. PRI=Primary Rate Interface
Troubleshooting ISDN Connections 15-3
ISDN: Dial Does Not Go Through (BRI)
ISDN: Dial Does Not Go Through (BRI)
Symptom: ISDN router using a Basic Rate Interface (BRI) port successfully dials, but the call does
not go through.
Table 15-2 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-2
ISDN: Dial Does Not Go Through (BRI)
Possible Problem
Solution
Speed setting mismatch
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check the dialer map interface configuration command entries in the
local and remote router. These entries will look similar to the following:
dialer map ip 131.108.2.5 speed 56 name C4000
Misconfigured dialer map
Number in use
Step 2
Compare the speed setting configured on the router interfaces to the speed of your
ISDN service. The speeds must be the same. To set the speed on the router, use the
speed 56 | 64 keyword in the dialer map command.
Step 3
If you do not know what the speed of your ISDN service is, contact your ISDN
provider. Long distance calls are usually 56 Kbps.
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Look for dialer map interface configuration command entries.
Step 2
Make sure that each dialer map contains the phone number of the remote BRI.
Step 3
If the phone number of the remote BRI is properly specified in each dialer map
statement but the dial does not go through, the first call failed and there are no
numbers left to try.
Step 4
Make sure that a phone number is configured, then clear the interface using the clear
interface privileged EXEC command and try dialing again.
Step 1
Turn on ISDN debugging using the following privileged EXEC commands:
C4000#debug isdn event
ISDN events debugging is on
C4000#debug isdn q931
ISDN Q931 packets debugging is on
Misconfigured SPIDs
Incorrect cable
Port not attached to proper device or port
15-4 Internetwork Troubleshooting Guide
Step 2
If the debug output says “User busy,” the remote ISDN number is probably in use.
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Look for an isdn spid1 spid-number interface configuration command
entry.
Step 2
Verify that the SPID specified in the command is that assigned to you by your service
provider.
Step 1
Make sure you use a straight-through RJ-45 cable. To check the cable, hold the RJ-45
cable ends side by side. If the pins are in the same order, the cable is straight-through.
If the order of the pins is reversed, the cable is rolled.
Step 2
If you are using a rolled cable, replace it with a straight-through cable.
Step 1
The ISDN BRI port of a router must be attached to an NT1 device. If the router does
not have an internal NT1, obtain and connect an NT1 to the BRI port. (The Cisco 1004
router has an internal NT1. An internal NT1 is optional in the Cisco 2524 and 2525
routers.)
Step 2
Make sure that the BRI or terminal adapter is attached to the S/T port of the NT1.
ISDN: Dial Does Not Go Through (BRI)
Possible Problem
Solution
Layer 1 logic states hung
Step 1
Check the status lights on the NT1. For information on interpreting the status lights,
refer to the hardware documentation for the NT1.
Step 2
If the NT1 status lights do not indicate a problem, check the NT1 for a switch to set the
ohm termination. If it is present, set the switch to 100 ohms.
Step 3
Power cycle the NT1.
Step 4
Check the output of the show isdn status privileged EXEC command. The command
output should say “Layer 1 active.”
Step 5
If the router still does not dial, clear the BRI interface using the clear interface bri
privileged EXEC command.
Step 6
Again check the output of the show isdn status command to see if Layer 1 is active.
Step 7
If Layer 1 is not active, contact your carrier to confirm the connection.
Media problem
For information on troubleshooting WAN media, refer to the appropriate chapter for your
media and WAN implementation elsewhere in this publication.
Hardware problem
Step 1
Use the show isdn status privileged EXEC command. The output of this command
should indicate “Layer 1 active.”
Step 2
If the output does not say “Layer 1 active,” verify that the configured switch type is
correct (check with your service provider to find out the correct switch type).
Step 3
Check the cable connecting the BRI or terminal adapter to the telco jack or NT1.
Replace the cable if it is damaged.
Step 4
Make sure the NT1 is functioning correctly. If there is faulty or malfunctioning
hardware, replace as necessary.
Step 5
Make sure that the router is functioning correctly. If there is faulty or malfunctioning
hardware, replace as necessary. For more information, refer to the “Troubleshooting
Hardware and Booting Problems” chapter.
Troubleshooting ISDN Connections 15-5
ISDN: Dial Does Not Go Through (PRI)
ISDN: Dial Does Not Go Through (PRI)
Symptom: ISDN router using a PRI port successfully dials, but the call does not go through.
Table 15-3 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-3
ISDN: Dial Does Not Go Through (PRI)
Possible Problem
Solution
Speed setting mismatch
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check the dialer map interface configuration command entries in the
local and remote router. These entries will look similar to the following:
dialer map ip 131.108.2.5 speed 56 name C4000
Step 2
Compare the speed setting configured on the router interfaces to the speed of your
ISDN service. The speeds must be the same. To set the speed on the router, use the
speed 56 | 64 keyword in the dialer map command.
Note: If the speed is not explicitly specified, the dialer map defaults to 64 Kbps.
Misconfigured dialer map
Number in use
Mismatched framing or linecoding
Step 3
If you do not know what the speed of your ISDN service is, contact your ISDN
provider. Long distance calls are usually 56 Kbps.
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Look for dialer map interface configuration command entries.
Step 2
Make sure that each dialer map contains the phone number of the remote PRI.
Step 3
If the phone number of the remote PRI is properly specified in each dialer map
statement but the dial does not go through, the first call failed and there are no
numbers left to try.
Step 4
Make sure that a phone number is configured, then clear the interface using the clear
interface privileged EXEC command and try dialing again.
Step 1
Turn on ISDN debugging using the debug isdn events privileged EXEC command.
Step 2
If the debug output says “User busy,” the remote ISDN number is probably in use.
Step 1
Use the show controllers t1 privileged EXEC command to see the framing and
linecoding types currently configured on the MIP1 card.
Step 2
Compare the configured framing and linecoding with those configured on the CSU.
(Refer to the vendor documentation for information on how to check the CSU
configuration.) The framing and linecoding configured on the MIP card and the CSU
must be the same.
Step 3
Change the framing or linecoding types as necessary to make them the same on the
MIP card and the CSU.
On the router, use the following controller configuration commands to configure the
framing and linecoding on the MIP card:
c7000(config)#controller t1 interface-number
c7000(config-controller)#framing [esf|sf]
c7000(config-controller)#linecode [ami|b8zs]
On the CSU, consult the vendor documentation for information on changing the
configuration.
15-6 Internetwork Troubleshooting Guide
ISDN: Dial Does Not Go Through (PRI)
Possible Problem
Solution
Incorrect cable
Step 1
Make sure you using a straight-through DB-15 cable.
Step 2
If you are using any other cable, replace it with a straight-through DB-15 cable.
Port not attached to proper device or port
The ISDN PRI port of a router must be attached to a CSU device. If the port is not connected to
a CSU, obtain a CSU and attach the PRI port to it.
Layer 1 logic states hung
Step 1
Check the status lights of the CSU. For information on interpreting the status lights,
refer to your vendor documentation.
Step 2
If the CSU status lights do not indicate a problem, power cycle the CSU.
Step 3
Check the output of the show isdn status privileged EXEC command. The command
output should say “Layer 1 active.”
Step 4
If the router still does not dial, clear the PRI interface using the clear interface serial
privileged EXEC command.
Step 5
Again check the output of the show isdn status command to see if Layer 1 is active.
Step 6
If Layer 1 is not active, contact your carrier to confirm the connection.
Media problem
For information on troubleshooting WAN media, refer to the appropriate chapter for your
media elsewhere in this publication.
Hardware problem
Step 1
Use the show isdn status privileged EXEC command. The output of this command
should indicate “Layer 1 active.”
Step 2
If the output does not say “Layer 1 active,” verify that the configured switch type is
correct (check with your service provider to find out the switch type).
Step 3
Check the cable connecting the PRI to the CSU. Replace the cable if it is damaged.
Step 4
Make sure that the router is functioning correctly. If there is faulty or malfunctioning
hardware, replace as necessary. For more information, refer to the “Troubleshooting
Hardware and Booting Problems” chapter.
1. MIP=MultiChannel Interface Processor
Troubleshooting ISDN Connections 15-7
ISDN: No Communication with Remote Router
ISDN: No Communication with Remote Router
Symptom: ISDN connection attempts are successful, but attempts to ping or otherwise
communicate with the remote ISDN router interface fail.
Table 15-4 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-4
ISDN: No Communication with Remote Router
Possible Problem
Solution
CHAP misconfigured
Step 1
Use the debug ppp chap privileged EXEC command.
Step 2
Try to ping the remote router. Look for the message, “Passed chap authentication.”
Step 3
If you do not see this message, use the show running-config privileged EXEC
command to view the router configuration. Make sure that the ppp authentication
chap interface configuration command is configured on both the local and remote
router.
Step 4
Check username global configuration command entries. Make sure that username
statements use the hostname of the remote router. Make sure that the passwords on
both the local and remote router are identical. Use the username command to add or
alter username entries. For more information, refer to the Cisco IOS configuration
guides and command references.
Step 1
Use the show running-config privileged EXEC command to view the interface state.
Check the output to see if the encapsulation ppp interface configuration command is
present.
Step 2
If PPP encapsulation is not configured, configure the interface with the encapsulation
ppp command.
Step 3
Verify that PPP encapsulation is being used by checking the show running-config
output again.
Step 1
Enter the show route privileged EXEC command for the particular protocol you are
using. For example, if you are using IP, enter show ip route ip-address. If the output
says “Network not in table” there is no route to the remote network.
Step 2
If there are no routes to remote networks, you need to add static routes using the
appropriate command for the protocol you are running. For example, to configure
static IP routes, use the ip route global configuration command.
Step 3
You also need to configure floating static routes, so that there will be routes to the
remote networks if the primary link goes down.
PPP encapsulation not configured on
interface
No route to remote network
For information on configuring floating static routes, refer to the Cisco IOS Wide-Area
Networking Configuration Guide and Wide-Area Networking Command Reference.
Misconfigured dialer map command
15-8 Internetwork Troubleshooting Guide
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Look for dialer map interface configuration command entries.
Step 2
Make sure that the dialer maps point to the correct next hop address. Also ensure that
the next hop address is in the same subnet as the local DDR interface address.
ISDN: No Communication with Remote Router
Possible Problem
Solution
Missing dialer-group command
Step 1
A dialer group must be configured on the local and remote router interfaces. Use the
show running-config privileged EXEC command to view the remote router
configuration. Look for a dialer-group interface configuration command entry.
Step 2
If the remote router interface has no dialer-group command entry, you must configure
a dialer group on the interface. Use the dialer-group group-number interface
configuration command. Make sure that the group number corresponds to the group
number referenced in dialer list command entries.
For more information, refer to the Cisco IOS Wide-Area Networking Configuration
Guide and Wide-Area Networking Command Reference.
Troubleshooting ISDN Connections 15-9
ISDN: No Communication End-to-End
ISDN: No Communication End-to-End
Symptom: ISDN connection attempts are successful, but attempts to ping or otherwise
communicate end-to-end over an ISDN connection are unsuccessful.
Table 15-5 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-5
ISDN: No Communication End-to-End
Possible Problem
Solution
No default gateway configured on end
systems
Step 1
Check the configuration of local and remote end systems. Make certain that end
systems are configured with a default-gateway specification.
Step 2
If end systems are not configured with a default gateway, you must configure one. For
information on configuring your end system, refer to the vendor documentation.
Step 3
If there is already a default gateway specification, make sure that it points to the
correct address.
The default gateway should point to a local router LAN interface.
No route to remote network
Step 1
Enter the show route privileged EXEC command for the particular protocol you are
using. For example, if you are using IP, enter show ip route ip-address. If the output
says “Network not in table” then there is no route to the remote network.
Step 2
If there are no routes to remote networks, you need to add static routes using the
appropriate command for the protocols you are running. For example, to configure
static IP routes, use the ip route global configuration command.
Step 3
You also need to configure floating static routes so there will be routes to the remote
networks after the primary link goes down.
For information on configuring floating static routes, refer to the Cisco IOS Wide-Area
Networking Configuration Guide and Wide-Area Networking Command Reference.
LAN media problem
Make certain that your LAN media is functioning properly and that addressing and other
configurations are correct. For more information on troubleshooting LAN problems, refer to the
“Troubleshooting LAN Media Problems” chapter.
Hardware problem
Step 1
Check all hardware on end systems (workstations and servers). Replace any damaged
or malfunctioning hardware.
Step 2
Check all router hardware. Replace any damaged or malfunctioning router hardware.
For more information, refer to the “Troubleshooting Hardware and Booting Problems”
chapter.
15-10 Internetwork Troubleshooting Guide
ISDN: Second B Channel Does Not Come Up
ISDN: Second B Channel Does Not Come Up
Symptom: When using a second B channel as a backup connection to a single destination, the
second B channel does not come up.
Table 15-6 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-6
ISDN: Second B Channel Does Not Come Up
Possible Problem
Solution
Missing or misconfigured dialer
load-threshold command
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check for a dialer load-threshold interface configuration command
entry.
Step 2
If the command is not present, configure the router interface with the dialer
load-threshold load command. This command specifies what the load on the first
B channel must be before the second B channel is activated.
Note: The range is from 1 to 255.
No dialer map configured for second
B channel
Step 3
If the command is already configured, make sure that the load value specified is not
too high. Reduce the specified load in increments of 25 or 50 to see if the second
channel will come up.
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Look for dialer map interface configuration command entries.
Step 2
If there is not a dialer map configured for the second remote ISDN telephone number,
configure one for the missing telephone number.
In some topologies (PRIs, or a BRI with a 5ESS switch), one telephone number refers
to both B channels. If this is the case, you will not be able to add a second dialer map
statement.
No SPID specified for second B channel
(BRI only)
No second B channel on remote router
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Look for isdn spid1 interface configuration command entries.
Step 2
Make sure that there are isdn spid1 command entries for each of the B channels. Make
sure that the specified SPIDs are those assigned to you by your service provider.
The remote ISDN router must have at least two B channels for the local router to be able to use
its second B channel. Contact your service provider to find out whether the remote ISDN router
has at least two B channels.
Troubleshooting ISDN Connections 15-11
ISDN: Second B Channel Comes Up Too Late
ISDN: Second B Channel Comes Up Too Late
Symptom: When using a second B channel as a backup connection to a single destination, the load
on the first B channel is higher than desired before the second B channel comes up.
Table 15-7 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-7
ISDN: Second B Channel Comes Up Too Late
Possible Problem
Solution
Misconfigured dialer load-threshold
command
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check the dialer load-threshold interface configuration command
entry.
Step 2
Make sure that the value configured by this command is not too high. This command
specifies what the load on the first B channel must be before the second B channel is
activated.
Note: The range is from 1 to 255.
Step 3
15-12 Internetwork Troubleshooting Guide
If the load value specified is too high, decrease the specified load in increments of 25
or 50 to allow the second B channel to dial earlier.
ISDN: Second B Channel Comes Up Too Early
ISDN: Second B Channel Comes Up Too Early
Symptom: When using a second B channel as a backup connection to a single destination, the
second B channel comes up before the load on the first B channel is high enough.
Table 15-8 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-8
ISDN: Second B Channel Comes Up Too Early
Possible Problem
Solution
Misconfigured dialer load-threshold
command
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check the dialer load-threshold interface configuration command
entry.
Step 2
Make sure that the value configured by this command is not too low. This command
specifies what the load on the first B channel must be before the second B channel is
activated.
Note: The range is from 1 to 255.
Step 3
If the load value specified is too low, increase the specified load in increments of 25 or
50 to allow the load on the first B channel to reach a greater value before the second
B channel dials.
Troubleshooting ISDN Connections 15-13
ISDN: Slow Performance
ISDN: Slow Performance
Symptom: ISDN connections are successfully established and communication occurs, but
performance across the link is slow.
Table 15-9 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-9
ISDN: Slow Performance
Possible Problem
Solution
Hold queues too small
Step 1
Check for input or output drops on the ISDN interface:
• For a BRI interface, use the show interfaces bri number 1 2 privileged EXEC
command
• For a PRI interface, use the show interfaces serial slot/port privileged EXEC
command
• For a serial interface, use the show interfaces serial number privileged EXEC
command
Look for drops in the command output. The output line looks similar to the following:
output queue 0/40 0 drops; input queue 0/75 0 drops
Step 2
If there are excessive drops on the interface, use the appropriate clear counters
privileged EXEC command to clear the interface counters. Check for drops on the
interface again. If the values are incrementing, you should increase the size of the
input or output hold queues.
Step 3
Increase the hold queue for the interface that is dropping packets. Use the hold-queue
length out or the hold-queue length in interface configuration command, depending
on whether you are seeing output or input drops on the interface.
Increase these queues by small increments (for instance, 25%) until you no longer see
drops in the show interfaces output.
Poor line quality
Step 1
Check for input or output errors on the ISDN interface.
• For a BRI interface, use the show interfaces bri number 1 2 privileged EXEC
command
• For a PRI interface, use the show interfaces serial slot/port privileged EXEC
command
• For a serial interface, use the show interfaces serial number privileged EXEC
command.
Look for errors in the command output. The output lines look similar to the following:
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, 2 interface resets
Step 2
If there are excessive errors on the interface, use the appropriate clear counters
privileged EXEC command to clear the interface counters. Check for errors on the
interface again. If the values are incrementing, it is probably the result of poor line
quality.
Step 3
Reduce the line speed to 56 Kbps to see if the error rate slows or stops.
Step 4
Contact your carrier to see if something can be done to improve the line quality. Make
sure the DCE device is configured properly as well.
15-14 Internetwork Troubleshooting Guide
ISDN: Line Disconnects Too Slowly
ISDN: Line Disconnects Too Slowly
Symptom: ISDN connections are successfully established but idle connections do not disconnect
quickly enough.
Table 15-10 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-10
ISDN: Line Disconnects Too Slowly
Possible Problem
Solution
No dialer hold-queue command configured
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check for a dialer hold-queue interface configuration command entry.
Step 2
Configure the dialer hold-queue packets command on the outgoing interface if it is
not present already. This command allows interesting outgoing packets to be queued
until a modem connection is established.
The number of packets specified by this command should be under 20.
Misconfigured dialer idle-timeout
command
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check for a dialer idle-timeout interface configuration command entry.
If the command is not present under the interface, the interface will use the default of
120 seconds.
dialer fast-idle time too high
Step 2
Check the value specified by this command. If the ISDN line disconnects too slowly
when idle, the value is probably set too high.
Step 3
Decrease the value specified by the dialer idle-timeout command. This will force
connections to disconnect more quickly when they are idle.
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check for a dialer fast-idle interface configuration command entry.
This command will not appear in the configuration unless it has been changed from
the default.
Dialer list access-list commands not
restrictive enough
Step 2
Check the value specified by this command. If there is contention for an ISDN line but
an idle connection does not disconnect quickly enough, the value is probably set too
high.
Step 3
Decrease the value specified by the dialer fast-idle command. This will force idle
connections to disconnect more quickly when there is contention for the line.
Step 1
Use the show access-list privileged EXEC command to see the access lists configured
on the router. Access lists determine which packets will cause dialing to occur and
which packets will reset the idle-timer, keeping the connection up.
Step 2
Use the show running-config privileged EXEC command to view the router
configuration. Check to see which access lists are applied to the interface with
dialer-list commands.
Step 3
Examine the specified access lists to make sure that the line is not being kept up for
uninteresting traffic. The access lists need to be more restrictive if the line never goes
down.
In particular, make sure that routing updates or SNMP packets do not reset the idle
timer or bring the line up.
Step 4
If necessary, modify access lists to restrict uninteresting traffic.
Troubleshooting ISDN Connections 15-15
ISDN: Line Disconnects Too Quickly
ISDN: Line Disconnects Too Quickly
Symptom: ISDN connections are successfully established but connections disconnect too quickly
when idle.
Table 15-11 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 15-11
ISDN: Line Disconnects Too Quickly
Possible Problem
Solution
Misconfigured dialer idle-timeout
command
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check for a dialer idle-timeout interface configuration command entry.
If the command is not present on the interface, the interface will use the default of
120 seconds.
dialer fast-idle time too high
Step 2
Check the value specified by this command. If the ISDN line disconnects too quickly
when idle, the value is probably set too low.
Step 3
Increase the value specified by the dialer idle-timeout command. This will allow
connections to stay idle longer before disconnecting.
Step 1
Use the show running-config privileged EXEC command to view the router
configuration. Check for a dialer fast-idle interface configuration command entry.
This command will not appear in the configuration unless it has been changed from
the default of 20 seconds.
Step 2
Check the value specified by this command. If there is contention for an ISDN line and
the line disconnects too quickly, the value is probably set too low.
Step 3
Increase the value specified by the dialer fast-idle command. This will allow idle
connections to stay connected longer when there is contention for the line.
15-16 Internetwork Troubleshooting Guide
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