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Table of Contents

Troubleshooting ISDN Connections

Troubleshooting ISDN Connections

Integrated Services Digital Network (ISDN) refers to a set of digital services that are becoming available to end users. ISDN involves the digitization of the telephone network so that voice, data, text, graphics, music, video, and other source material can be provided to end users from a single end-user terminal over existing telephone wiring. Proponents of ISDN imagine a worldwide network much like the present telephone network, but with digital transmission and a variety of new services.

ISDN is an effort to standardize subscriber services, user/network interfaces, and network and internetwork capabilities. Standardizing subscriber services attempts to ensure a level of international compatibility. Standardizing the user/network interface stimulates development and marketing of these interfaces by third-party manufacturers. Standardizing network and internetwork capabilities helps achieve the goal of worldwide connectivity by ensuring that ISDN networks easily communicate with one another.

ISDN applications include high-speed image applications (such as Group IV facsimile), additional telephone lines in homes to serve the telecommuting industry, high-speed file transfer, and video conferencing. Voice, of course, will also be a popular application for ISDN.

Many carriers are beginning to offer ISDN under tariff. In North America, large local-exchange carriers (LECs) are beginning to provide ISDN service as an alternative to the T1 connections (digital carrier facilities provided by telephone companies) that currently carry bulk wide-area telephone service (WATS) services.

ISDN Components

ISDN components include terminals, terminal adapters (TAs), network-termination devices, line-termination equipment, and exchange-termination equipment. ISDN terminals come in two types. Specialized ISDN terminals are referred to as terminal equipment type 1 (TE1). Non-ISDN terminals such as DTE that predate the ISDN standards are referred to as terminal equipment type 2 (TE2). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link. TE2s connect to the ISDN network through a terminal adapter. The ISDN TA can either be a standalone device or a board inside the TE2. If the TE2 is implemented as a standalone device, it connects to the TA via a standard physical-layer interface. Examples include EIA/TIA-232-C (formerly RS-232-C), V.24, and V.35.

Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the network termination type 1 (NT1) or network termination type 2 (NT2) device. These are network-termination devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. In North America, the NT1 is a customer premises equipment (CPE) device. In most other parts of the world, the NT1 is part of the network provided by the carrier. The NT2 is a more complicated device, typically found in digital private branch exchanges (PBXs), that performs Layer 2 and 3 protocol functions and concentration services. An NT1/2 device also exists; it is a single device that combines the functions of an NT1 and an NT2.

A number of reference points are specified in ISDN. These reference points define logical interfaces between functional groupings such as TAs and NT1s. ISDN reference points include the following:

A sample ISDN configuration is shown in Figure 17-1. This figure shows three devices attached to an ISDN switch at the central office. Two of these devices are ISDN compatible, so they can be attached through an S reference point to NT2 devices. The third device (a standard, non-ISDN telephone) attaches through the R reference point to a TA. Any of these devices could also attach to an NT1/2 device, which would replace both the NT1 and the NT2. And, although they are not shown, similar user stations are attached to the far right ISDN switch.


Figure 17-1: A Sample ISDN Configuration


ISDN Services

The ISDN Basic Rate Interface (BRI) service offers two B channels and one D channel (2B+D). BRI B-channel service operates at 64 kbps and is meant to carry user data; BRI D-channel service operates at 16 kbps and is meant to carry control and signaling information, although it can support user data transmission under certain circumstances. The D-channel signaling protocol comprises Layers 1 through 3 of the OSI reference model. BRI also provides for framing control and other overhead, bringing its total bit rate to 192 kbps. The BRI physical layer specification is International Telecommunication Union Telecommunication Standardization Sector (ITU-T; formerly the Consultative Committee for International Telegraph and Telephone [CCITT]) I.430.

ISDN Primary Rate Interface (PRI) service offers 23 B channels and one D channel in North America and Japan, yielding a total bit rate of 1.544 Mbps (the PRI D channel runs at 64 kbps). ISDN PRI in Europe, Australia, and other parts of the world provides 30 B plus one 64-kbps D channel and a total interface rate of 2.048 Mbps. The PRI physical layer specification is ITU-T I.431.

Layer 1

ISDN physical layer (Layer 1) frame formats differ depending on whether the frame is outbound (from terminal to network) or inbound (from network to terminal). Both physical layer interfaces are shown in Figure 17-2.


Figure 17-2: ISDN Physical-Layer Frame Formats


The frames are 48 bits long, of which 36 bits represent data. The bits of an ISDN physical layer frame are used as follows:

Multiple ISDN user devices can be physically attached to one circuit. In this configuration, collisions can result if two terminals transmit simultaneously. ISDN therefore provides features to determine link contention. When an NT receives a D bit from the TE, it echoes back the bit in the next E-bit position. The TE expects the next E bit to be the same as its last transmitted D bit.

Terminals cannot transmit into the D channel unless they first detect a specific number of ones (indicating "no signal") corresponding to a preestablished priority. If the TE detects a bit in the echo (E) channel that is different from its D bits, it must stop transmitting immediately. This simple technique ensures that only one terminal can transmit its D message at one time. After successful D message transmission, the terminal has its priority reduced by being required to detect more continuous ones before transmitting. Terminals cannot raise their priority until all other devices on the same line have had an opportunity to send a D message. Telephone connections have higher priority than all other services, and signaling information has a higher priority than nonsignaling information.

Layer 2

Layer 2 of the ISDN signaling protocol is Link Access Procedure on the D channel, also known as LAPD. LAPD is similar to High-Level Data Link Control (HDLC) and Link Access Procedure, Balanced (LAPB). As the expansion of the LAPD abbreviation indicates, it is used across the D channel to ensure that control and signaling information flows and is received properly. The LAPD frame format (see Figure 17-3) is very similar to that of HDLC and, like HDLC, LAPD uses supervisory, information, and unnumbered frames. The LAPD protocol is formally specified in ITU-T Q.920 and ITU-T Q.921.


Figure 17-3: LAPD Frame Format


The LAPD Flag and Control fields are identical to those of HDLC. The LAPD Address field can be either 1 or 2 bytes long. If the extended address bit of the first byte is set, the address is 1 byte; if it is not set, the address is 2 bytes. The first address field byte contains the service access point identifier (SAPI), which identifies the portal at which LAPD services are provided to Layer 3. The C/R bit indicates whether the frame contains a command or a response. The terminal endpoint identifier (TEI) field identifies either a single terminal or multiple terminals. A TEI of all ones indicates a broadcast.

Layer 3

Two Layer 3 specifications are used for ISDN signaling: ITU-T (formerly CCITT) I.450 (also known as ITU-T Q.930) and ITU-T I.451 (also known as ITU-T Q.931). Together, these protocols support user-to-user, circuit-switched, and packet-switched connections. A variety of call establishment, call termination, information, and miscellaneous messages are specified, including SETUP, CONNECT, RELEASE, USER INFORMATION, CANCEL, STATUS, and DISCONNECT.     These messages are functionally similar to those provided by the X.25 protocol (see "Troubleshooting X.25 Connections," for more information). Figure 17-4, from ITU-T I.451, shows the typical stages of an ISDN circuit-switched call.


Figure 17-4:
ISDN Circuit-Switched Call Stages


Troubleshooting ISDN

This section presents troubleshooting information for ISDN connectivity and performance problems. It describes specific ISDN symptoms, the problems that are likely to cause each symptom, and the solutions to those problems.

The following sections describe the most common network issues when working with ISDN:

ISDN: Router Does Not Dial

Symptom: Router configured for ISDN does not dial.

Table 17-1 outlines the problems that might cause this symptom and describes solutions to those problems.


Table 17-1: 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 "Troubleshooting Hardware and Booting Problems," and "Troubleshooting Serial Line Problems."

Missing or misconfigured dialer map commands

Step 1 Use the show running-config privileged exec command to view the router configuration. Check whether 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.

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
    

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.

For detailed information on configuring dialer maps, refer to the Cisco IOS Wide-Area Networking Configuration Guide and Wide-Area Networking Command Reference.

No dialer group configured

Step 1 Use the show running-config privileged exec command to view the router configuration. Check whether 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 by using the dialer-group group-number interface configuration command. This command associates an interface with a dialer group.

Step 3 Make sure the group-number is the same number used in the associated dialer-list global configuration commands.

Missing or misconfigured dialer lists

Step 1 Use the show running-config privileged exec command to view the router configuration. Check whether 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 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.

Missing or misconfigured access lists

Step 1 Use the show running-config privileged exec command to view the router configuration. Check whether 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 does not occur. Configure access lists that define interesting traffic and make sure the lists are referenced correctly by dialer-list commands.

In the following example, IGRP1 routing updates are classified as uninteresting (they do not cause the router to dial), whereas 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 If access lists are 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.

Missing pri-group command

On Cisco 7000 series routers, use the pri-group controller configuration command to specify ISDN PRI2 on a channelized T1 card.

Step 1 Use the show running-config privileged exec command to view the router configuration. Check whether 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 of a 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
    
1IGRP = Interior Gateway Routing Protocol
2PRI = Primary Rate Interface

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 17-2 outlines the problems that might cause this symptom and describes solutions to those problems.


Table 17-2: 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 should 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 interface 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 the speed of your ISDN service, contact your ISDN provider. Long-distance calls are usually 56 kbps.

For detailed information on configuring dialer maps, refer to the Cisco IOS Wide-Area Networking Configuration Guide and Wide-Area Networking Command Reference.

Misconfigured dialer map

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, and then clear the interface using the clear interface privileged exec command and try dialing again.

For detailed information on configuring dialer maps, refer to the Cisco IOS Wide-Area Networking Configuration Guide and Wide-Area Networking Command Reference.

Number in use

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
    

Caution: Because debugging output is assigned high priority in the CPU process, it can render the system unusable. For this reason, use debug commands only to troubleshoot specific problems or during troubleshooting sessions with Cisco technical support staff. Moreover, it is best to use debug commands during periods of lower network traffic and fewer users. Debugging during these periods decreases the likelihood that increased debug command processing overhead will affect system use.

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

Misconfigured SPIDs1

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. Use the set spid command to enter the correct spid:

set spid

set [spid id] spid [spid number]

Syntax Description:

  • spid id---(Optional if there is only one SPID) Used as a convenient single-digit number to identify the actual long SPIDs allocated by service providers.
  • spid number---Number identifying the service to which you have subscribed. This value is assigned by the ISDN service provider and is usually a 10-digit telephone number with some extra digits. The SPID number can consist of 1 to 20 digits.

Misconfigured SPID

Examples:

The following example sets two SPIDs for the line:

    Host> set 1 spid 0408555123401
    Host> set 2 spid 0405555123402
    

The following example deletes the first SPID:

    Host> set 1 spid
    

Incorrect cable

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.

Port not attached to proper device or port

Step 1 The ISDN BRI port of a router must be attached to an NT12 device. In ISDN, NT1 is a device that provides the interface between the customer premises equipment and central office switching equipment. 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.

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 the switch is present, set it 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 whether 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 book.

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 "Troubleshooting Hardware and Booting Problems."

1SPID = service profile identifier
2NT1 = network termination type 1

ISDN: Dial Does Not Go Through PRI

Symptom: ISDN router using a PRI port successfully dials, but the call does not go through.

Table 17-3 outlines the problems that might cause this symptom and describes solutions to those problems.


Table 17-3:
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 should 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.

Step 3 If you do not know the speed of your ISDN service, contact your ISDN provider. Long-distance calls are usually 56 kbps.

For detailed information on configuring dialer maps, refer to the Cisco IOS Wide-Area Networking Configuration Guide and Wide-Area Networking Command Reference.

Misconfigured dialer map

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 a phone number is configured, and then clear the interface using the clear interface privileged exec command and try dialing again.

For detailed information on configuring dialer maps, refer to the Cisco IOS Wide-Area Networking Configuration Guide and Wide-Area Networking Command Reference.

Number in use

Step 1 Turn on ISDN debugging by 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.

Mismatched framing or linecoding

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 CSU2. (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.

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 whether 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 book.

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 determine 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 "Troubleshooting Hardware and Booting Problems."

1MIP = MultiChannel Interface Processor
2CSU = channel service unit

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 17-4 outlines the problems that might cause this symptom and describes solutions to those problems.


Table 17-4: ISDN: No Communication with Remote Router

Possible Problem Solution

CHAP1 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 routers.

Syntax:

ppp authentication {chap | chap pap | pap chap | pap} [if-needed] [list-name | default] [callin] [one-time]

  • chap---Enables CHAP on a serial interface.
  • pap---Enables PAP2 on a serial interface.
  • chap pap---Enables both CHAP and PAP, and performs CHAP authentication before PAP.
  • pap chap---Enables both CHAP and PAP, and performs PAP authentication before CHAP.
  • if-needed---(Optional) Used with TACACS3 and extended TACACS. Does not perform CHAP or PAP authentication if the user has already provided authentication. This option is available only on asynchronous interfaces.

CHAP misconfigured

  • list-name---(Optional) Used with AAA4. Specifies the name of a list of methods of authentication to use. If no list name is specified, the system uses the default. The list is created with the aaa authentication ppp command.
  • default---The name of the method list is created with the aaa authentication ppp command.
  • callin---Specifies authentication on incoming (received) calls only.
  • one-time---(Optional) Accepts the username and password in the username field.

Step 4 Check username global configuration command entries. Make sure that username statements use the host name 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.

PPP5 encapsulation not configured on interface

Step 1 Use the show running-config privileged exec command to view the interface state. Check the output to see whether 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.

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," 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.

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

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 DDR6 interface address.

For detailed information on configuring dialer maps, refer to the Cisco IOS Wide-Area Networking Configuration Guide and Wide-Area Networking Command Reference.

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 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.

1CHAP = Challenge Handshake Authentication Protocol
2PAP = Password Authentication Protocol
3TACACS = Terminal Access Controller Access Control System
4AAA = authentication, authorization, and accounting
5PPP = Point-to-Point Protocol
6DDR = dial-on-demand routing

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 17-5 outlines the problems that might cause this symptom and describes solutions to those problems.


Table 17-5: 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 an end systems is 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 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 are functioning properly and that addressing and other configurations are correct. For more information on troubleshooting LAN problems, refer to the media troubleshooting chapter that covers the media type used in your network.

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 "Troubleshooting Hardware and Booting Problems."

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 17-6 outlines the problems that might cause this symptom and describes solutions to those problems.


Table 17-6: 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.

The following is the syntax for the dialer load-threshold command:

dialer load-threshold load [outbound | inbound | either]

This command defines the load level that must be exceeded on the first ISDN B channel before the router attempts to bring up a second B channel for a multilink PPP connection. The load is a value between 1 and 255 and defines a fraction taken over 255. The load can be calculated based on outbound, inbound, or either inbound or outbound traffic on the interface. For instance, the sample configurations use "dialer load-threshold 200 either." If the combination of both inbound and outbound traffic levels reaches 200/255 (about 80%, or roughly 50 kbps) capacity of the first ISDN B channel, the router attempts to bring up a second B channel to assist with the traffic load. Thus, the second
B channel is activated only when the traffic demands exceed the capacity of one B channel.

Note: In scenarios where it is desired to have both B channels all the time, use the value 1. This causes both
B channels to be brought up whenever the router dials.

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 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 whether the second channel comes up.

No dialer map configured for second B channel

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.

For detailed information on configuring dialer maps, refer to the Cisco IOS Wide-Area Networking Configuration Guide and Wide-Area Networking Command Reference.

No service profile handler (SPID) specified for second B channel (BRI only)

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.

No second B channel on remote router

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.

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 17-7 outlines the problem that might cause this symptom and describes solutions to that problem.


Table 17-7: 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.

The following is the syntax for the dialer load-threshold command:

dialer load-threshold load [outbound | inbound | either]

This command defines the load level that must be exceeded on the first ISDN B channel before the router attempts to bring up a second B channel for a multilink PPP connection. The load is a value between 1 and 255 and defines a fraction taken over 255. The load can be calculated based on outbound, inbound, or either inbound or outbound traffic on the interface. For instance, the sample configurations use "dialer load-threshold 200 either." If the combination of both inbound and outbound traffic levels reaches 200/255 (about 80%, or roughly 50 kbps) capacity of the first ISDN B channel, the router attempts to bring up a second B channel to assist with the traffic load. Thus, the second B channel is activated only when the traffic demands exceed the capacity of one B channel.

Note: In scenarios where it is desired to have both B channels all the time, use the value 1. This causes both B channels to be brought up whenever the router dials.

Step 2 Make sure 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 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.

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 17-8 outlines the problem that might cause this symptom and describes solutions to that problem.


Table 17-8: 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.

The following is the syntax for the dialer load-threshold command:

dialer load-threshold load [outbound | inbound | either]

This command defines the load level that must be exceeded on the first ISDN B channel before the router attempts to bring up a second B channel for a multilink PPP connection. The load is a value between 1 and 255 and defines a fraction taken over 255. The load can be calculated based on outbound, inbound, or either inbound or outbound traffic on the interface. For instance, the sample configurations use "dialer load-threshold 200 either." If the combination of both inbound and outbound traffic levels reaches 200/255 (about 80%, or roughly 50 kbps) capacity of the first ISDN B channel, the router attempts to bring up a second B channel to assist with the traffic load. Thus, the second B channel is activated only when the traffic demands exceed the capacity of one B channel.

Note: In scenarios where it is desired to have both B channels all the time, use the value 1. This causes both B channels to be brought up whenever the router dials.

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 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.

ISDN: Slow Performance

Symptom: ISDN connections are successfully established and communication occurs, but performance across the link is slow.

Table 17-9 outlines the problems that might cause this symptom and describes solutions to those problems.


Table 17-9: 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 whether the error rate slows or stops.

Step 4 Contact your carrier to see whether something can be done to improve the line quality. Make sure the DCE device is configured properly as well.

ISDN: Line Disconnects Too Slowly

Symptom: ISDN connections are successfully established but idle connections do not disconnect quickly enough.

Table 17-10 outlines the problems that might cause this symptom and describes solutions to those problems.


Table 17-10: 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 less than 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.

Syntax:

dialer idle-timeout seconds

This command specifies the number of seconds before the router disconnects an ISDN call due to lack of interesting traffic as defined by the dialer-list command. Whenever an interesting packet is forwarded over the ISDN line, the dial idle-timeout counter resets to 0 and begins counting up again. When the counter reaches the specified value, the router terminates the call. For proper operation, both routers should have matching idle-timeout values. The default value is 120 seconds but can be configured to a value between 1 and 2,147,483 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 forces connections to disconnect more quickly when they are idle.

dialer fast-idle time too high

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 does not appear in the configuration unless it has been changed from the default.

Syntax:

dialer fast-idle seconds

The fast-idle timer is activated if there is contention for
a line. In other words, if a line is in use and a packet for a different next hop address is received, and the busy line is required to send the competing packet, the dialer fast-idle timer is activated.

Step 2 Check the value specified by the dialer fast-idle 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 forces idle connections to disconnect more quickly when there is contention for the line.

Dialer list access-list commands not restrictive enough

Step 1 Use the show access-list privileged exec command to see the access lists configured on the router. Access lists determine which packets cause dialing to occur and which packets reset the idle-timer, keeping the connection up.

Step 2 Use the show running-config privileged exec command to view the router configuration. Check which access lists are applied to the interface with dialer-list commands.

Syntax:

dialer-list dialer-group protocol protocol {permit | deny | list access-list-number}

This command creates a list that defines what traffic is interesting enough to initiate and sustain an ISDN call for an interface that belongs to the same dialer-group number.

Step 3 Examine the specified access lists to make sure 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 SNMP1 packets do not reset the idle timer or bring the line up.

Step 4 If necessary, modify access lists to restrict uninteresting traffic.

1SNMP = Simple Network Management Protocol

ISDN: Line Disconnects Too Quickly

Symptom: ISDN connections are successfully established, but connections disconnect too quickly when idle.

Table 17-11 outlines the problems that might cause this symptom and describes solutions to those problems.


Table 17-11: 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 uses 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 allows connections to stay idle longer before disconnecting.

dialer fast-idle time
too high

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.

The fast-idle timer is activated if there is contention for a line. In other words, if a line is in use and a packet for a different next hop address is received, and the busy line is required to send the competing packet, the dialer fast-idle timer is activated.

This command does 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 allows idle connections to stay connected longer when there is contention for the line.


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Posted: Tue May 16 15:12:27 PDT 2000
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