Troubleshooting ISDN Connections

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 2 If there is not a dialer map configured for the protocol you are using, create a dialer map for each protocol.

For example, if you want to configure a dialer map for IP, enter dialer map commands similar to the following on the interface:

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 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 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 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.

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:

Step 3 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.

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

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.

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 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.

C4000#debug isdn event
ISDN events debugging is on
C4000#debug isdn q931
ISDN Q931 packets debugging is on

Step 2 If the debug output says "User busy," the remote ISDN number is probably in use.

Step 2 Verify that the SPID specified in the command is that assigned to you by your service provider.

Step 2 If you are using a rolled cable, replace it with a straight-through cable.

Step 2 Make sure that the BRI or terminal adapter is attached to the S/T port of 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.

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.

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.

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 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 2 If the debug output says "User busy," the remote ISDN number is probably in use.

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.

Step 2 If you are using any other cable, replace it with a straight-through DB-15 cable.

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.

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.

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 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 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.

For information on configuring floating static routes, refer to the Cisco IOS Wide-Area Networking Configuration Guide and Wide-Area Networking Command Reference.

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.

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.

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.

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.

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.

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.

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 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.

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.

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 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.

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.

• 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.

• 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.

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.

If the command is not present under the interface, the interface will use the default of 120 seconds.

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.

This command will not appear in the configuration unless it has been changed from the default.

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 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.

If the command is not present on the interface, the interface will use the default of 120 seconds.

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.

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.