Dialup Technology: Troubleshooting Techniques
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Contents
Introduction
Dialup is simply the application of the public switched telephone network (PSTN) that carries data on behalf of the end user. It involves a customer premises equipment (CPE) device sending the telephone switch a phone number to which to direct a connection. The Cisco3600, AS5200, AS5300, and AS5800 are all examples of routers that have the ability to run a PRI along with banks of digital modems. The AS2511, on the other hand, is an example of a router that communicates with external modems.
Prerequisites
Requirements
Readers of this document should be knowledgeable of the following:
The carrier market has grown significantly, and the market now demands higher modem densities. The answer to this need is a higher degree of interoperation with the telephone company equipment and the development of the digital modem. This is a modem that is capable of direct digital access to the PSTN. As a result, faster CPE modems have now been developed that take advantage of the clarity of signal that the digital modems enjoy. The fact that the digital modems connecting into the PSTN through a PRI or BRI can transmit data at over 53k using the V.90 communication standard, attests to the success of the idea.
The first access servers were the Cisco2509 and Cisco2511. The AS2509 could support 8 incoming connections using external modems, and the AS2511 could support 16. The AS5200 was introduced with 2 PRIs and could support 48 users using digital modems, and it represented a major leap forward in technology. Modem densities have increased steadily with the AS5300 supporting 4 and then 8 PRIs. Finally, the AS5800 was introduced to fill the needs of carrier class installations needing to handle dozens of incoming T1s and hundreds of user connections.
A couple of outdated technologies bear mentioning in a historical discussion of dialer technology. 56Kflex is an older (pre-V.90) 56k modem standard that was proposed by Rockwell. Cisco supports version 1.1 of the 56Kflex standard on its internal modems, but recommends migrating the CPE modems to V.90 as soon as possible. Another outdated technology is the AS5100. The AS5100 was a joint venture between Cisco and a modem manufacturer. The AS5100 was created as a way to increase modem density through the use of quad modem cards. It involved a group of AS2511s built as cards that inserted into a backplane shared by quad modem cards, and a dual T1 card.
Components Used
This document is not restricted to specific software and hardware versions.
The information presented in this document was created from devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If you are working in a live network, ensure that you understand the potential impact of any command before using it.
Conventions
For more information on document conventions, refer to the Cisco Technical Tips Conventions.
Troubleshooting Incoming Calls
Troubleshooting an incoming call starts at the bottom and works its way up. The general flow of reasoning looks for the following:
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Do we see the call arrive? (A yes answer advances to the next question)
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Does the receiving end answer the call?
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Does the call complete?
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Is data passing across the link?
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Is the session established? (PPP or terminal)
For modem connections, a data call looks the same as a terminal session coming in until the end where the data call goes to negotiate PPP.
For incoming calls involving digital modems, first make sure the underlying ISDN or CAS is receiving the call. If using an external modem, the ISDN and CAS group sections can be skipped.
Incoming ISDN Call Troubleshooting
Use the command debug isdn q931. Here's an example output from a successful connection:
Router# debug isdn q931 RX <- SETUP pd = 8 callref = 0x06 Bearer Capability i = 0x8890 Channel ID i = 0x89 Calling Party Number i = 0x0083, `5551234' TX -> CONNECT pd = 8 callref = 0x86 RX <- CONNECT_ACK pd = 8 callref = 0x06
The setup message indicates that a connection is being initiated by the remote end. The call reference numbers are maintained as a pair. In this case the call reference number for the incoming side of the connection is 0x06, and the call reference number of the outbound side of the connection is 0x86. The Bearer Capability (often referred to as the bearercap) tells the router what kind of call is coming in. In this case the connection is type 0x8890. That value indicates "ISDN Speed 64 Kb/s". If the bearercap had been 0x8090A2, it would have indicated "Speech/voice call u-law".
If no setup message came in, you should verify the correct number by calling it manually, if it is voice provisioned. You should also check the status of the ISDN interface (refer to Using the show isdn status Command for BRI Troubleshooting). If that all checks out, make sure that the call originator is making the correct call. This can be done by contacting the telephone company. The call originator can trace the call to see where it?s being sent. If the connection is long distance, try a different long distance carrier using a 1010 long distance code.
If the call coming in is an async modem call, make sure the line is provisioned to allow voice calls.
Note: BRI async modem calling is a feature of 3600 routers running 12.0(3)T, or later. It requires a recent hardware revision of the BRI interface network module. WIC modules do not support async modem calling.
If the call arrived but did not complete, look for a cause code (see Table 17-10). A successful completion is indicated by connect-ack.
If this is an async modem call, move forward to the "Incoming Modem Call Troubleshooting" section.
At this point the ISDN call is connected, but no data has been seen coming across the link. Use the command debug ppp negotiate to see if any PPP traffic is coming across the line. If you do not see traffic, there may be a speed mismatch. To determine if this is the case, 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
For dialer profiles, a map-class needs to be defined in order to set the speed. Note that, by default, ISDN interfaces attempt to use 64K communications speeds on each channel.
For detailed information on configuring dialer maps and profiles, refer to the Cisco IOS Dial Solutions Configuration Guide, Dial Solutions Command Reference, and the Dial Solutions Quick Configuration Guide.
If you receive valid PPP packets, the link is up and working. You should proceed to the "Troubleshooting PPP" section at this time.
Incoming CAS Call Troubleshooting
To troubleshoot the CAS group serving connectivity to the modems, use the commands debug modem, debug modem csm, and debug cas.
Note: The debug cas command first appeared in 12.0(7)T for the AS5200 and AS5300. Earlier versions of IOS use the system level configuration command service internal along with the exec command modem-mgmt debug rbs. Debugging this information on an AS5800 requires connecting to the trunk card itself.
First, determine if the telephone company switch went "offhook" to signal the incoming call. If it did not, verify the number being called. Do this by attaching a phone to the originating side's phone line and calling the number. If the call comes in properly, the problem is in the originating CPE. If the call still does not show up on the CAS, check the T1 (chapter 15).In this instance, use the debug serial interfaces command.
The following shows a good connection using debug modem CSM:
Router# debug modem csm CSM_MODEM_ALLOCATE: slot 1 and port 0 is allocated. MODEM_REPORT(0001): DEV_INCALL at slot 1 and port 0 CSM_PROC_IDLE: CSM_EVENT_ISDN_CALL at slot 1, port 0 CSM_RING_INDICATION_PROC: RI is on CSM_RING_INDICATION_PROC: RI is off CSM_PROC_IC1_RING: CSM_EVENT_MODEM_OFFHOOK at slot 1, port 0 MODEM_REPORT(0001): DEV_CONNECTED at slot 1 and port 0 CSM_PROC_IC2_WAIT_FOR_CARRIER: CSM_EVENT_ISDN_CONNECTED at slot 1, port 0
In this example, the call was directed to a modem. If your call was directed to a modem, proceed to the "Incoming Modem Call Troubleshooting" section, below.
Incoming Modem Call Troubleshooting
Use the following debug commands when troubleshooting incoming modem calls:
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debug modem
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debug modem csm (for integrated digital modems)
Use the following debug commands in conjunction to indicate the new call coming in:
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debug isdn q931
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debug cas
Assuming the call reaches the modem, the modem needs to pick the call up.
Tips for debugging External Modems
To facilitate debugging on an external modem connected to a TTY line, increase the speaker volume. This helps to make some problems more apparent.
When the originating modem calls, does the receiving modem ring? If not, verify the number and try a manual call from the remote site. Try using a regular phone on the receiving end as well. Replace cables and hardware as needed.
Async Modem Call Pickup
If an external modem is not answering, check the cabling between the modem and the access server or router. Confirm that the modem is connected to the TTY or auxiliary port on the router with a rolled RJ-45 cable and an MMOD DB-25 adapter. Cisco recommends and supports this cable configuration for RJ-45 ports. Note that these connectors are typically labeled: Modem.
RJ-45 cabling comes in a few types: straight, rolled, and crossover. You can determine the cabling type by holding the two ends of an RJ-45 cable side-by-side. You'll see eight colored strips, or pins, at each end.
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If the order of the colored pins is the same at each end, the cable is straight.
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If the order of the colors is reversed at each end, the cable is rolled.
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The cable is a crossover cable if colors indicate the following:
RJ45 to RJ45 crossover cable:
RJ45 RJ45
5 ------------------ 2
2 ------------------ 5
4 ------------------ 1
1 ------------------ 4
To make sure the signaling is OK, use the show line command outlined in chapter 16.
Cabling issues aside, an external modem needs to be initialized to auto-answer. Check the remote modem to see whether it is set to auto-answer. Usually, an AA indicator light is on when auto-answer is set. Set the remote modem to auto-answer if it is not already set. For information on verifying and changing the modem's settings, refer to your modem documentation. Use a reverse telnet to initialize the modem (refer to chapter 16).
Digital (Integrated) Modem Call Pickup
On an external modem it is clear whether the call is being answered, but internal modems require a manual call to the receiving number. Listen for the answer back tone (ABT). If you do not hear an ABT, check the configuration for the following two things:
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Make sure the command isdn incoming-voice modem exists under any ISDN interfaces handling incoming modem connections.
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Under the line configuration for the modem's TTY, make sure the command modem inout exists.
It is also possible that the Call Switching Module (CSM) did not allocate an internal modem to handle the incoming call. This problem can be caused by modem or resource pools being configured for too few incoming connections. It may also mean that the access server may simply be out of modems. Check the availability of modems and adjust the modem pool or resource pool manager settings appropriately. If a modem was allocated and the configuration shows modem inout, gather debugs and contact Cisco for assistance.
Modem Trainup
If the receiving modem raises DSR, the trainup was successful. Trainup failures can indicate a circuit problem or modem incompatibility.
To get to the bottom of an individual modem problem, go to the AT prompt at the originating modem while it's attached to the POTS line of interest. If calling into a digital modem in a Cisco access server, be prepared to record a .wav file of the trainup music, or digital impairment learning sequence (DIL). The DIL is the musical score (PCM sequence) that the originating V.90 analog modem tells the receiving digital modem to play back. The sequence allows the analog modem to discern any digital impairment in the circuit; such as multiple D/A conversions, a law/u-law, robbed bits, or digital pads. If you don't hear the DIL, the modems did not negotiate V.90 in V.8/V.8bis (that is., a modem compatibility issue). If you do hear the DIL and a retrain in V.34, the analog modem decided (on the basis of the DIL playback) that V.90 was infeasible.
Does the music have noise in it? If so, then clean up the circuit.
Does the client give up quickly, without running V.34 training? For example, perhaps it doesn't know what to do when it hears V.8bis. In this case you should try disabling V.8bis (hence K56Flex) on the server (if acceptable). You should get new client firmware or swap out the client modem. Alternately, the dialing end could insert five commas at the end of the dial string. This delays the calling modem's listen and will cause the V.8bis tone from the receiving server to timeout without affecting the client modem. Five commas in the dial string is a general guideline and might need adjusting to allow for local conditions.
Session Establishment
At this point in the sequence, the modems are connected and trained up. Now it's time to find out if any traffic is coming across properly.
If the line receiving the call is configured with autoselect ppp and the async interface is configured with async mode interactive, use the command debug modem to verify the autoselect process. As traffic comes in over the async link, the access server will examine the traffic to determine whether the traffic is character-based or packet-based. Depending on the determination, the access server will then either start a PPP session or go no farther than having an exec session on the line.
A normal autoselect sequence with inbound PPP LCP packets:
*Mar 1 21:34:56.958: TTY1: DSR came up *Mar 1 21:34:56.962: tty1: Modem: IDLE->READY *Mar 1 21:34:56.970: TTY1: EXEC creation *Mar 1 21:34:56.978: TTY1: set timer type 10, 30 seconds *Mar 1 21:34:59.722: TTY1: Autoselect(2) sample 7E !--- The inbound traffic is displayed in hexadecimal format. This is based on the !--- bits coming in over the line, regardless of whether the bits are ASCII !--- characters or elements of a packet. The bits represented in this example are !--- correct for a LCP packet. Anything different would be either a malformed packet !--- or character traffic. *Mar 1 21:34:59.726: TTY1: Autoselect(2) sample 7EFF *Mar 1 21:34:59.730: TTY1: Autoselect(2) sample 7EFF7D *Mar 1 21:34:59.730: TTY1: Autoselect(2) sample 7EFF7D23 *Mar 1 21:34:59.734: TTY1 Autoselect cmd: ppp negotiate !--- Having determined that the inbound traffic is actually an LCP packet, the access !--- server triggers the PPP negotiation process. *Mar 1 21:34:59.746: TTY1: EXEC creation *Mar 1 21:34:59.746: TTY1: create timer type 1, 600 seconds *Mar 1 21:34:59.794: TTY1: destroy timer type 1 (OK) *Mar 1 21:34:59.794: TTY1: destroy timer type 0 *Mar 1 21:35:01.798: %LINK-3-UPDOWN: Interface Async1, changed state to up !--- The async interface changes state to up, and the PPP negotiation (not shown) !--- commences.
If the call is a PPP session and if async mode dedicated is configured on the async interface, use the command debug ppp negotiation to see if any configuration request packets are coming from the remote end. The debugs show these as CONFREQ. If you observe both inbound and outbound PPP packets, proceed to "Troubleshooting PPP". Otherwise, connect from the call-originating end with a character-mode (or "exec") session (that is, a non-PPP session).
Note: If the receiving end displays async modem dedicated under the async interface, an exec dial-in only shows what appears to be random ASCII garbage. To allow a terminal session and still have PPP capability, use the async interface configuration command async mode interactive. Under the associated line's configuration, use the command autoselect ppp.
Modem Cannot Send or Receive Data
If the modems connect with a terminal session and no data comes across, check the following possible causes and suggested courses of action:
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Modem speed setting is not locked
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Use the show line exec command on the access server or router. The output for the auxiliary port should indicate the currently configured Tx and Rx speeds.
For an explanation of the output of the show line command, see the "Using Debug Commands" section in chapter 15.
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If the line is not configured to the correct speed, use the speed line configuration command to set the line speed on the access server or router line. Set the value to the highest speed in common between the modem and the access server or router port. To set the terminal baud rate, use the speed line configuration command. This command sets both the transmit (to terminal) and receive (from terminal) speeds.
Syntax:
speed bps
Syntax Description:
bps - Baud rate in bits per second (bps). The default is 9600 bps.
The following example sets lines 1 and 2 on a Cisco 2509 access server to 115200 bps:
line 1 2 speed 115200
Note: If, for some reason, you cannot use flow control, limit the line speed to 9600 bps. Faster speeds are likely to result in lost data.
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Use the show line exec command again and confirm that the line speed is set to the desired value.
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When you are certain that the access server or router line is configured for the desired speed, initiate a reverse Telnet session to the modem via that line. For more information, see the section "Establishing a Reverse Telnet Session to a Modem" in chapter 16.
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Use a modem command string that includes the "lock DTE speed" command for your modem. See your modem documentation for exact configuration command syntax.
Note: The lock DTE speed command, which might also be referred to as port rate adjust or buffered mode, is often related to the way in which the modem handles error correction. This command varies widely from one modem to another.
Locking the modem speed ensures that the modem always communicates with the Cisco access server or router at the speed configured on the Cisco auxiliary port. If this command is not used, the modem reverts to the speed of the data link (the telephone line), instead of communicating at the speed configured on the access server.
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Hardware flow control not configured on local or remote modem or router
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Use the show line aux-line-number exec command and look for the following in the Capabilities field:
Capabilities: Hardware Flowcontrol In, Hardware Flowcontrol Out
For more information, refer to Interpreting Show Line Output in Chapter 16.
If there is no mention of hardware flow control in this field, hardware flow control is not enabled on the line. Hardware flow control for access server-to-modem connections is recommended.
For an explanation of the output of the show line command, see the section "Using Debug Commands" in chapter 15.
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Configure hardware flow control on the line using the flowcontrol hardware line configuration command.
To set the method of data flow control between the terminal or other serial device and the router, use the flowcontrol line configuration command. Use the no form of this command to disable flow control.
Syntax:
flowcontrol {none | software [lock] [in | out] | hardware [in | out]}
Syntax Description:
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none - Turns off flow control.
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software - Sets software flow control. An optional keyword specifies the direction: in causes the Cisco IOS software to listen to flow control from the attached device, and out causes the software to send flow control information to the attached device. If you do not specify a direction, both are assumed.
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lock - Makes it impossible to turn off flow control from the remote host when the connected device needs software flow control. This option applies to connections using the Telnet or rlogin protocols.
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hardware - Sets hardware flow control. An optional keyword specifies the direction: in causes the software to listen to flow control from the attached device, and out causes the software to send flow control information to the attached device. If you do not specify a direction, both are assumed. For more information about hardware flow control, see the hardware manual that was shipped with your router.
Example:
The following example sets hardware flow control on line 7:
line 7 flowcontrol hardware
Note: If for some reason you cannot use flow control, limit the line speed to 9600 bps. Faster speeds are likely to result in lost data.
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After enabling hardware flow control on the access server or router line, initiate a reverse Telnet session to the modem via that line. For more information, see the section "Establishing a Reverse Telnet Session to a Modem" in chapter 16.
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Use a modem command string that includes the RTS/CTS Flow command for your modem. This command ensures that the modem is using the same method of flow control (that is, hardware flow control) as the Cisco access server or router. See your modem documentation for exact configuration command syntax.
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Misconfigured dialer map commands
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Use the show running-config privileged exec command to view the router configuration. Check the dialer map command entries to see whether the broadcast keyword is specified.
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If the keyword is missing, add it to the configuration.
Syntax:
dialer map protocol next-hop-address [name hostname] [broadcast] [dial-string]
Syntax Description:
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protocol - The protocol subject to mapping. Options include IP, IPX, bridge, and snapshot.
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next-hop-address - The protocol address of the opposite site's async interface.
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name hostname - A required parameter used in PPP authentication. It is the name of the remote site for which the dialer map is created. The name is case sensitive and must match the hostname of the remote router.
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broadcast - An optional keyword that broadcast packets (for example, IP RIP or IPX RIP/SAP updates) that is forwarded to the remote destination. In static routing sample configurations, routing updates are not desired and the broadcast keyword is omitted.
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dial-string - The remote site's phone number. Any access codes (for example, 9 to get out of an office, international dialing codes, area codes) must be included.
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Make sure that dialer map commands specify the correct next hop addresses.
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If the next hop address is incorrect, change it using the dialer map command.
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Make sure all other options in dialer map commands are correctly specified for the protocol you are using.
For detailed information on configuring dialer maps, refer to the Cisco IOS Wide-Area Networking Configuration Guide and Wide-Area Networking Command Reference.
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Problem with dialing modem
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Make sure that the dialing modem is operational and is securely connected to the correct port. Determine if another modem works when connected to the same port.
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Debugging an incoming exec session generally falls into a few main categories:
Dialup Client Receives No exec Prompt
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Autoselect is enabled on the line
Attempt to access exec mode by pressing Enter.
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Line is configured with the no exec command
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Use the show line exec command to view the status of the appropriate line.
Check the Capabilities field to see if it says "exec suppressed." If this is the case, the no exec line configuration command is enabled.
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Configure the exec line configuration command on the line to allow exec sessions to be initiated. This command has no arguments or keywords.
The following example turns on the exec on line 7:
line 7 exec
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Flow control is not enabled.
or
Flow control is enabled only on one device (either DTE or DCE).
or
Flow control is misconfigured.
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Use the show line aux-line-number exec command and look for the following in the Capabilities field:
Capabilities: Hardware Flowcontrol In, Hardware Flowcontrol Out
For more information, refer to Interpreting Show Line Output in Chapter 16.
If there is no mention of hardware flow control in this field, hardware flow control is not enabled on the line. Hardware flow control for access server-to-modem connections is recommended.
For an explanation of the output from the show line command, see the "Using Debug Commands" section in chapter 15.
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Configure hardware flow control on the line using the flowcontrol hardware line configuration command. The following example sets hardware flow control on line 7:
line 7 flowcontrol hardware
Note: If for some reason you cannot use flow control, limit the line speed to 9600 bps. Faster speeds are likely to result in lost data.
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After enabling hardware flow control on the access server or router line, initiate a reverse Telnet session to the modem via that line. For more information, see the section "Establishing a Reverse Telnet Session to a Modem" in chapter 16.
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Use a modem command string that includes the RTS/CTS Flow command for your modem. This command ensures that the modem is using the same method of flow control (that is, hardware flow control) as the Cisco access server or router. See your modem documentation for exact configuration command syntax.
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Modem speed setting is not locked
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Use the show line exec command on the access server or router. The output for the auxiliary port should indicate the currently configured Tx and Rx speeds.
For an explanation of the output of the show line command, see the "Using Debug Commands" section in chapter 15.
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If the line is not configured to the correct speed, use the speed line configuration command to set the line speed on the access server or router line. Set the value to the highest speed in common between the modem and the access server or router port. To set the terminal baud rate, use the speed line configuration command. This command sets both the transmit (to terminal) and receive (from terminal) speeds.
Syntax:
speed bps
Syntax Description:
bps - Baud rate in bits per second (bps). The default is 9600 bps.
Example:
The following example sets lines 1 and 2 on a Cisco 2509 access server to 115200 bps:
line 1 2 speed 115200
Note: If for some reason you cannot use flow control, limit the line speed to 9600 bps. Faster speeds are likely to result in lost data.
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Use the show line exec command again and confirm that the line speed is set to the desired value.
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When you are certain that the access server or router line is configured for the desired speed, initiate a reverse Telnet session to the modem via that line. For more information, see the section "Establishing a Reverse Telnet Session to a Modem" in chapter 16.
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Use a modem command string that includes the lock DTE speed command for your modem. See your modem documentation for exact configuration command syntax.
Note: The lock DTE speed command, which might also be referred to as port rate adjust or buffered mode, is often related to the way in which the modem handles error correction. This command varies widely from one modem to another.
Locking the modem speed ensures that the modem always communicates with the Cisco access server or router at the speed configured on the Cisco auxiliary port. If this command is not used, the modem reverts to the speed of the data link (the telephone line) instead of communicating at the speed configured on the access server.
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Dialup Sessions Sees "Garbage"
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Modem speed setting is not locked
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Use the show line exec command on the access server or router. The output for the auxiliary port should indicate the currently configured Tx and Rx speeds.
For an explanation of the output of the show line command, see the "Using Debug Commands" section in chapter 15.
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If the line is not configured to the correct speed, use the speed line configuration command to set the line speed on the access server or router line. Set the value to the highest speed in common between the modem and the access server or router port.
To set the terminal baud rate, use the speed line configuration command. This command sets both the transmit (to terminal) and receive (from terminal) speeds.
Syntax:
speed bps
Syntax Description:
bps Baud rate in bits per second (bps). The default is 9600 bps.
Example:
The following example sets lines 1 and 2 on a Cisco 2509 access server to 115200 bps:
line 1 2
speed 115200
Note: If for some reason you cannot use flow control, limit the line speed to 9600 bps. Faster speeds are likely to result in lost data.
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Use the show line exec command again and confirm that the line speed is set to the desired value.
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When you are certain that the access server or router line is configured for the desired speed, initiate a reverse Telnet session to the modem via that line. For more information, see the section "Establishing a Reverse Telnet Session to a Modem" in chapter 16.
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Use a modem command string that includes the lock DTE speed command for your modem. See your modem documentation for exact configuration command syntax.
Note: The lock DTE speed command, which might also be referred to as port rate adjust or buffered mode, is often related to the way in which the modem handles error correction. This command varies widely from one modem to another.
Locking the modem speed ensures that the modem always communicates with the Cisco access server or router at the speed configured on the Cisco auxiliary port. If this command is not used, the modem reverts to the speed of the data link (the telephone line) instead of communicating at the speed configured on the access server.
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Symptom: Remote dialin session opens up in an already-existing session initiated by another user. That is, instead of getting a login prompt, a dialin user sees a session established by another user (which might be a UNIX command prompt, a text editor session, and so forth).
Dialup Session Opens in Existing Session
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Modem configured for DCD always high
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The modem should be reconfigured to have DCD high only on CD. This is usually accomplished by using the &C1 modem command string, but check your modem documentation for the exact syntax for your modem.
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You might have to configure the access server line to which the modem is connected with the no exec line configuration command. Clear the line with the clear line privileged exec command, initiate a reverse Telnet session with the modem, and reconfigure the modem so that DCD is high only on CD.
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End the Telnet session by entering disconnect and reconfigure the access server line with the exec line configuration command
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Modem control is not enabled on the access server or router
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Use the show line exec command on the access server or router. The output for the auxiliary port should be show inout or RIisCD in the Modem column. This indicates that modem control is enabled on the line of the access server or router.
For an explanation of the show line output, see the "Using Debug Commands" section in chapter 15.
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Configure the line for modem control using the modem inout line configuration command. Modem control is now enabled on the access server.
Note: Be certain to use the modem inout command instead of the modem dialin command while the connectivity of the modem is in question. The latter command allows the line to accept incoming calls only. Outgoing calls will be refused, making it impossible to establish a Telnet session with the modem to configure it. If you want to enable the modem dialin command, do so only after you are certain the modem is functioning correctly.
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Incorrect cabling
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Check the cabling between the modem and the access server or router. Confirm that the modem is connected to the auxiliary port on the access server or router with a rolled RJ-45 cable and an MMOD DB-25 adapter. This cabling configuration is recommended and supported by Cisco for RJ-45 ports. These connectors are typically labeled: Modem.
There are two types of RJ-45 cabling: straight and rolled. If you hold the two ends of an RJ-45 cable side-by-side, you'll see eight colored strips, or pins, at each end. If the order of the colored pins is the same at each end, then the cable is straight. If the order of the colors is reversed at each end, then the cable is rolled.
The rolled cable (CAB-500RJ) is standard with Cisco's 2500/CS500.
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Use the show line exec command to verify that the cabling is correct. See the explanation of the show line command output in the section "Using Debug Commands" in this chapter 15.
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Dialup Receiving Modem Does Not Disconnect Properly
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Modem is not sensing DTR
Enter the Hangup DTR modem command string. This command tells the modem to drop carrier when the DTR signal is no longer being received.
On a Hayes-compatible modem the &D3 string is commonly used to configure Hangup DTR on the modem. For the exact syntax of this command, see the documentation for your modem.
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Modem control is not enabled on the router or access server
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Use the show line exec command on the access server or router. The output for the auxiliary port should show inout or RIisCD in the Modem column. This indicates that modem control is enabled on the line of the access server or router.
For an explanation of the show line output, see the "Using Debug Commands" section in chapter 15.
-
Configure the line for modem control using the modem inout line configuration command. Modem control is now enabled on the access server.
Note: Be certain to use the modem inout command instead of the modem dialin command while the connectivity of the modem is in question. The latter command allows the line to accept incoming calls only. Outgoing calls will be refused, making it impossible to establish a Telnet session with the modem to configure it. If you want to enable the modem dialin command, do so only after you are certain the modem is functioning correctly.
-
Troubleshooting Outbound Calls
While the troubleshooting approach for incoming calls starts at the bottom, troubleshooting an outbound connection starts at the top. The general flow of reasoning looks for the following:
-
Does the Dial on Demand Routing (DDR) initiate a call? (A yes answer advances to the next question)
-
If this is an async modem, do the chat scripts issue the expected commands?
-
Does the call make it out to the PSTN?
-
Does the remote end answer the call?
-
Does the call complete?
-
Is data passing over the link?
-
Is the session established? (PPP or Terminal)
Verifying Dialer Operation
To see if the dialer is trying to make a call to its remote destination, use the command debug dialer events. More detailed information can be gained from debug dialer packet, but the debug dialer packet command is resource intensive and should not be used on a busy system that has multiple dialer interfaces operating.
The following line of debug dialer events output for an IP packet lists the name of the DDR interface and the source and destination addresses of the packet:
Dialing cause: Async1: ip (s=172.16.1.111 d=172.16.2.22)
If traffic does not initiate a dial attempt, the most common reason is improper configuration (either of the interesting traffic definitions, the state of the dialer interface, or the routing).
Traffic Does Not Initiate a Dial Attempt
-
Missing or incorrect "interesting traffic" definitions
-
Using the command show running-config, ensure that the interface is configured with a dialer-group and that there is a global level dialer-list configured with a matching number.
-
Ensure that the dialer-list command is configured to permit either an entire protocol or to permit traffic matching an access list
-
Verify that the access-list declares packets going across the link to be interesting. One useful test is to use the privileged exec command debug ip packet [list number] using the number of the pertinent access list. Then attempt to ping, or otherwise send traffic, across the link. If the interesting traffic filters have been properly defined, you will see the packets in the debug output. If there is no debug output from this test, then the access-list is not matching the packets.
-
-
Interface state
Use the command show interfaces [interface name] to ensure that the interface is in the state "up/up (spoofing)".
-
Interface in "standby" mode
Another (primary) interface on the router has been configured to use the dialer interface as a backup interface. Furthermore, the primary interface is not in a state of "down/down", which is required to bring the dialer interface out of standby mode. Also, a backup delay must be configured on the primary interface, or the backup interface command will never be enforced.
To check that the dialer interface will change from "standby" to "up/up (spoofing)", it is usually necessary to pull the cable from the primary interface. Simply shutting down the primary interface with the configuration command shutdown will not put the primary interface into "down/down", but instead will put it into "administratively down"-not the same thing.
In addition, if the primary connection is via Frame Relay, the Frame Relay configuration must be done on a point-to-point Serial sub-interface, and the telephone company must be passing the "Active" bit. This practice is also known as "end-to-end LMI".
-
Interface is "administratively down"
The dialer interface has been configured with the command shutdown. This is also the default state of any interface when a Cisco router is booted for the very first time. Use the interface configuration command no shutdown to remove this impediment.
-
-
Incorrect routing
Issue the exec command show ip route [a.b.c.d], where a.b.c.d is the address of the dialer interface of the remote router. If ip unnumbered is used on the remote router, use the address of the interface listed in the ip unnumbered command.
The output should show a route to the remote address via the dialer interface. If there is no route, ensure that static or floating static routes have been configured by examining the output of show running-config.
If there is a route via an interface other than the dialer interface, the implication is that DDR is being used as a backup. Examine the router configuration to make sure that static or floating static routes have been configured. The surest way to test the routing, in this case, is to disable the primary connection and execute the show ip route [a.b.c.d] command to verify that the proper route has been installed in the routing table.
Note: If you attempt this during live network operations, a dial event may be triggered. This sort of testing is best accomplished during scheduled maintenance cycles.
Placing the Call
If the routing and the interesting traffic filters are correct, a call should be initiated. This can be seen by using debug dialer events:
Async1 DDR: Dialing cause ip (s=10.0.0.1, d=10.0.0.2) Async1 DDR: Attempting to dial 5551212
If the dialing cause is seen but no attempt is made to dial, the usual reason is a misconfigured dialer map or dialer profile.
Call Not Placed
Some possible problems and suggested actions are listed below:
-
Misconfigured dialer map
Use the command show running-config to ensure that the dialing interface is configured with at least one dialer map statement which points to the protocol address and called number of the remote site.
-
Misconfigured dialer profile
Use the command show running-config to ensure that the Dialer interface is configured with a dialer pool X command and that a dialer interface on the router is configured with a matching dialer pool-member X. If dialer profiles are not properly configured, you may see a debug message like:
Dialer1: Can't place call, no dialer pool set
Make sure that a dialer string is configured.
Async Outbound Calling - Verify Chat Script Operation
If the outbound call is a modem call, a chat script must execute in order for the call to proceed. For dialer map-based DDR, the chat script is invoked by the modem-script parameter in a dialer map command. If the DDR is dialer profile-based, this is accomplished by the command script dialer, configured on the TTY line. Both uses rely on a chat script existing in the router's global configuration, for example:
chat-script callout AT OK atdt\T TIMEOUT 60 CONNECT \c
In either event, the command to view the chat script activity is debug chat. If the dial string (that is, phone number) used in the dialer map or dialer string command were 5551212, the debug output would look like the following:
CHAT1: Attempting async line dialer script CHAT1: Dialing using Modem script: callout & System script: none CHAT1: process started CHAT1: Asserting DTR CHAT1: Chat script callout started CHAT1: Sending string: AT CHAT1: Expecting string: OK CHAT1: Completed match for expect: OK CHAT1: Sending string: atdt5551212 CHAT1: Expecting string: CONNECT CHAT1: Completed match for expect: CONNECT CHAT1: Chat script callout finished, status = Success
Chat script problems can be broken into three categories:
-
Configuration error
-
Modem failure
-
Connection failure
Chat Script Failure
This list shows possible outputs from debug chat shows and suggested actions:
-
no matching chat script found for [number]
A chat script has not been configured. Add one.
-
Chat script dialout finished, status = Connection timed out; remote host not responding
The modem is not responding to the chat script. Verify communication with the modem (refer to Table 16-2 in Chapter 16).
-
Timeout expecting: CONNECT
-
Possibility 1: The local modem is not actually placing the call. Verify that the modem can place a call by performing a reverse Telnet to the modem and manually initiating a dial.
-
Possibility 2: The remote modem is not answering. Test this by dialing the remote modem with an ordinary POTS telephone.
-
Possibility 3: The number being dialed is incorrect. Verify the number by dialing it manually. Correct the configuration, if necessary.
-
Possibility 4: The modem trainup is taking too long or the TIMEOUT value is too low. If the local modem is external, turn up the modem speaker volume and listen to the trainup tones. If the trainup is abruptly cut off, try increasing the TIMEOUT value in the chat-script command. If the TIMEOUT is already 60 seconds or more, see the Modem Trainup section.
-
ISDN Outbound Calling
Upon the first suspicion of an ISDN failure, either on a BRI or a PRI, always check the output from show isdn status. The key things to note are that Layer 1 should be Active and Layer 2 should be in a state of MULTIPLE_FRAME_ESTABLISHED. See the "Interpreting Show ISDN Status Output" section in Chapter 16 for information on reading this output, as well as for corrective measures.
For outbound ISDN calls, debug isdn q931 and debug isdn events are the best tools to use. Fortunately, debugging outbound calls is very similar to debugging incoming calls. A normal successful call might look like this:
*Mar 20 21:07:45.025: ISDN BR0: Event: Call to 5553759 at 64 Kb/s
*Mar 20 21:07:45.033: ISDN BR0: TX -> SETUP pd = 8 callref = 0x2C
*Mar 20 21:07:45.037: Bearer Capability i = 0x8890
*Mar 20 21:07:45.041: Channel ID i = 0x83
*Mar 20 21:07:45.041: Keypad Facility i = 0x35353533373539
*Mar 20 21:07:45.141: ISDN BR0: RX <- CALL_PROC pd = 8 callref = 0xAC
*Mar 20 21:07:45.145: Channel ID i = 0x89
*Mar 20 21:07:45.157: ISDN BR0: received HOST_PROCEEDING
Channel ID i = 0x0101
*Mar 20 21:07:45.161: -------------------
Channel ID i = 0x89
*Mar 20 21:07:45.313: ISDN BR0: RX <- CONNECT pd = 8 callref = 0xAC
*Mar 20 21:07:45.325: ISDN BR0: received HOST_CONNECT
!--- The CONNECT message is the key indicator of success. If a CONNECT is not received, !--- you may see a DISCONNECT or a RELEASE_COMP (release complete) message followed by !--- a cause code (see below)
*Mar 20 22:11:03.212: ISDN BR0: RX <- RELEASE_COMP pd = 8 callref = 0x8F
*Mar 20 22:11:03.216: Cause i = 0x8295 - Call rejected
The cause value indicates two things.
-
The second byte of the 4- or 6-byte value indicates from where in the end-to-end call path the DISCONNECT or RELEASE_COMP was received. This can help you to localize the problem.
-
The third and fourth bytes indicate the actual reason for the failure. See the tables which follow for the meanings of the different values.
Note: The following printout usually indicates a higher-protocol failure:
Cause i = 0x8090 - Normal call clearing
PPP authentication failure is a typical reason. Turn on debug ppp negotiation and debug ppp authentication before assuming that the connection failure is necessarily an ISDN problem
Cause Code Fields
Table 17-9 lists the ISDN cause code fields that display in the following format within the debug commands:
i=0x y1 y2 z1 z2 [a1 a2]
ISDN Cause Code Fields
| Field | Value Description |
|---|---|
| 0x | The values that follow are in hexadecimal. |
| y1 | 8--ITU-T standard coding. |
| y2 | 0--User 1--Private network serving local user 2--Public network serving local user 3--Transit network 4--Public network serving remote user 5--Private network serving remote user 7--International network A--Network beyond internetworking point |
| z1 | Class (the more significant hexadecimal number) of cause value. Refer to the next table for detailed information about possible values. |
| z2 | Value (the less significant hexadecimal number) of cause value. Refer to the next table for detailed information about possible values. |
| a1 | (Optional) Diagnostic field that is always 8. |
| a2 | (Optional) Diagnostic field that is one of the following values: 0--Unknown 1--Permanent 2--Transient |
ISDN Cause Values
The following table lists descriptions of some of the most commonly-seen cause values of the cause information element - the third and fourth bytes of the cause code. For more complete information about ISDN codes and values, refer to Understanding debug isdn q931 Disconnect Cause Codes.
| Hex Value | Cause | Explanation |
|---|---|---|
| 81 | Unallocated (unassigned) number | The ISDN number was sent to the switch in the correct format; however, the number is not assigned to any destination equipment. |
| 90 | Normal call clearing | Normal call clearing has occurred. |
| 91 | User busy | The called system acknowledges the connection request but is unable to accept the call because all B channels are in use. |
| 92 | No user responding | The connection cannot be completed because the destination does not respond to the call. |
| 93 | No answer from user (user alerted) | The destination responds to the connection request but fails to complete the connection within the prescribed time. The problem is at the remote end of the connection. |
| 95 | Call rejected | The destination is capable of accepting the call but rejected it for an unknown reason. |
| 9C | Invalid number format | The connection could be not established because the destination address was presented in an unrecognizable format or because the destination address was incomplete. |
| 9F | Normal, unspecified | Reports the occurrence of a normal event when no standard cause applies. No action required. |
| A2 | No circuit/channel available | The connection cannot be established because no appropriate channel is available to take the call. |
| A6 | Network out of order | The destination cannot be reached because the network is not functioning correctly, and the condition might last for an extended period of time. An immediate reconnect attempt will probably be unsuccessful. |
| AC | Requested circuit/channel not available | The remote equipment cannot provide the requested channel for an unknown reason. This might be a temporary problem. |
| B2 | Requested facility not subscribed | The remote equipment supports the requested supplementary service by subscription only. This is frequently a reference to long-distance service. |
| B9 | Bearer capability not authorized | The user requested a bearer capability that the network provides, but the user is not authorized to use it. This might be a subscription problem. |
| D8 | Incompatible destination | Indicates that an attempt was made to connect to non-ISDN equipment. For example, to an analog line. |
| E0 | Mandatory information element is missing | The receiving equipment received a message that did not include one of the mandatory information elements. This is usually due to a D-channel error. If this error occurs systematically, report it to your ISDN service provider. |
| E4 | Invalid information element contents | The remote equipment received a message that includes invalid information in the information element. This is usually due to a D-channel error. |
CAS Outbound Calling
For outbound calling via CAS T1 or E1 and integrated digital modems, much of the troubleshooting is similar to other DDR troubleshooting. The same holds true, as well, for outbound integrated modem calls over a PRI line. The unique features involved in making a call in this manner require special debugging in the event of a call failure.
As for other DDR situations, you must ensure that a call attempt is demanded. Use debug dialer events for this purpose. Refer to Verifying Dialer Operation.
Before a call can be placed, a modem must be allocated for the call. To view this process, and the subsequent call, use the following debug commands:
-
debug modem
-
debug modem csm
-
debug cas
Note: The debug cas command first appeared in IOS version 12.0(7)T for the AS5200 and AS5300. Earlier versions of IOS use a system-level configuration command service internal along with the exec command modem-mgmt debug rbs:
Turning on the Debugs
router#conf t Enter configuration commands, one per line. End with CNTL/Z. router(config)#service internal router(config)#^Z router#modem-mgmt csm ? debug-rbs enable rbs debugging no-debug-rbs disable rbs debugging router#modem-mgmt csm debug-rbs router# neat msg at slot 0: debug-rbs is on neat msg at slot 0: special debug-rbs is on
Turning off the Debugs
router# router#modem-mgmt csm no-debug-rbs neat msg at slot 0: debug-rbs is off
Note: Debugging this information on an AS5800 requires connecting to the trunk card. The following is an example of a normal outbound call over a CAS T1 that is provisioned and configured for FXS-Ground-Start:
Mica Modem(1/0): Rcvd Dial String(5551111) [Modem receives digits from chat script] CSM_PROC_IDLE: CSM_EVENT_MODEM_OFFHOOK at slot 1, port 0 CSM_RX_CAS_EVENT_FROM_NEAT:(A003): EVENT_CHANNEL_LOCK at slot 1 and port 0 CSM_PROC_OC4_DIALING: CSM_EVENT_DSX0_BCHAN_ASSIGNED at slot 1, port 0 Mica Modem(1/0): Configure(0x1) Mica Modem(1/0): Configure(0x2) Mica Modem(1/0): Configure(0x5) Mica Modem(1/0): Call Setup neat msg at slot 0: (0/2): Tx RING_GROUND Mica Modem(1/0): State Transition to Call Setup neat msg at slot 0: (0/2): Rx TIP_GROUND_NORING [Telco switch goes OFFHOOK] CSM_RX_CAS_EVENT_FROM_NEAT:(A003): EVENT_START_TX_TONE at slot 1 and port 0 CSM_PROC_OC5_WAIT_FOR_CARRIER: CSM_EVENT_DSX0_START_TX_TONE at slot 1, port 0 neat msg at slot 0: (0/2): Tx LOOP_CLOSURE [Now the router goes OFFHOOK] Mica Modem(1/0): Rcvd Tone detected(2) Mica Modem(1/0): Generate digits:called_party_num=5551111 len=8 Mica Modem(1/0): Rcvd Digits Generated CSM_PROC_OC5_WAIT_FOR_CARRIER: CSM_EVENT_ADDR_INFO_COLLECTED at slot 1, port 0 CSM_RX_CAS_EVENT_FROM_NEAT:(A003): EVENT_CHANNEL_CONNECTED at slot 1 and port 0 CSM_PROC_OC5_WAIT_FOR_CARRIER: CSM_EVENT_DSX0_CONNECTED at slot 1, port 0 Mica Modem(1/0): Link Initiate Mica Modem(1/0): State Transition to Connect Mica Modem(1/0): State Transition to Link Mica Modem(1/0): State Transition to Trainup Mica Modem(1/0): State Transition to EC Negotiating Mica Modem(1/0): State Transition to Steady State Mica Modem(1/0): State Transition to Steady State Speedshifting Mica Modem(1/0): State Transition to Steady State
Debugs for T1s and E1s with other signaling types are similar.
Getting to this point in the debugging indicates that the calling and answering modems have trained and connected, and that higher-layer protocols can begin to negotiate. If a modem is properly allocated for the outbound call but the connection fails to get this far, the T1 must be examined. Refer to Chapter 15 for T1 troubleshooting information.
Troubleshooting PPP
Troubleshooting the PPP portion of a connection begins when you know that the dial connection, ISDN or async, successfully establishes.
It is important to understand what a successful debug PPP sequence looks like before you troubleshoot PPP negotiation. In this way, comparing a faulty PPP debug session against a successfully-completed debug PPP sequence saves you time and effort.
Following is an example of a successful PPP sequence. See PPP LCP Negotiation Details for a detailed description of the output fields.
Montecito# Mar 13 10:57:13.415: %LINK-3-UPDOWN: Interface Async1, changed state to up Mar 13 10:57:15.415: As1 LCP: O CONFREQ [ACKrcvd] id 2 len 25 Mar 13 10:57:15.415: As1 LCP: ACCM 0x000A0000 (0x0206000A0000) Mar 13 10:57:15.415: As1 LCP: AuthProto CHAP (0x0305C22305) Mar 13 10:57:15.415: As1 LCP: MagicNumber 0x1084F0A2 (0x05061084F0A2) Mar 13 10:57:15.415: As1 LCP: PFC (0x0702) Mar 13 10:57:15.415: As1 LCP: ACFC (0x0802) Mar 13 10:57:15.543: As1 LCP: I CONFACK [REQsent] id 2 len 25 Mar 13 10:57:15.543: As1 LCP: ACCM 0x000A0000 (0x0206000A0000) Mar 13 10:57:15.543: As1 LCP: AuthProto CHAP (0x0305C22305) Mar 13 10:57:15.543: As1 LCP: MagicNumber 0x1084F0A2 (0x05061084F0A2) Mar 13 10:57:15.543: As1 LCP: PFC (0x0702) Mar 13 10:57:15.547: As1 LCP: ACFC (0x0802) Mar 13 10:57:16.919: As1 LCP: I CONFREQ [ACKrcvd] id 4 len 23 Mar 13 10:57:16.919: As1 LCP: ACCM 0x000A0000 (0x0206000A0000) Mar 13 10:57:16.919: As1 LCP: MagicNumber 0x001327B0 (0x0506001327B0) Mar 13 10:57:16.919: As1 LCP: PFC (0x0702) Mar 13 10:57:16.919: As1 LCP: ACFC (0x0802) Mar 13 10:57:16.919: As1 LCP: Callback 6 (0x0D0306) Mar 13 10:57:16.919: As1 LCP: O CONFREJ [ACKrcvd] id 4 len 7 Mar 13 10:57:16.919: As1 LCP: Callback 6 (0x0D0306) Mar 13 10:57:17.047: As1 LCP: I CONFREQ [ACKrcvd] id 5 len 20 Mar 13 10:57:17.047: As1 LCP: ACCM 0x000A0000 (0x0206000A0000) Mar 13 10:57:17.047: As1 LCP: MagicNumber 0x001327B0 (0x0506001327B0) Mar 13 10:57:17.047: As1 LCP: PFC (0x0702) Mar 13 10:57:17.047: As1 LCP: ACFC (0x0802) Mar 13 10:57:17.047: As1 LCP: O CONFACK [ACKrcvd] id 5 len 20 Mar 13 10:57:17.047: As1 LCP: ACCM 0x000A0000 (0x0206000A0000) Mar 13 10:57:17.047: As1 LCP: MagicNumber 0x001327B0 (0x0506001327B0) Mar 13 10:57:17.047: As1 LCP: PFC (0x0702) Mar 13 10:57:17.047: As1 LCP: ACFC (0x0802) Mar 13 10:57:17.047: As1 LCP: State is Open Mar 13 10:57:17.047: As1 PPP: Phase is AUTHENTICATING, by this end Mar 13 10:57:17.047: As1 CHAP: O CHALLENGE id 1 len 28 from "Montecito" Mar 13 10:57:17.191: As1 CHAP: I RESPONSE id 1 len 30 from "Goleta" Mar 13 10:57:17.191: As1 CHAP: O SUCCESS id 1 len 4 Mar 13 10:57:17.191: As1 PPP: Phase is UP Mar 13 10:57:17.191: As1 IPCP: O CONFREQ [Closed] id 1 len 10 Mar 13 10:57:17.191: As1 IPCP: Address 172.22.66.23 (0x0306AC164217) Mar 13 10:57:17.303: As1 IPCP: I CONFREQ [REQsent] id 1 len 40 Mar 13 10:57:17.303: As1 IPCP: CompressType VJ 15 slots CompressSlotID (0x0206002D0F01) Mar 13 10:57:17.303: As1 IPCP: Address 0.0.0.0 (0x030600000000) Mar 13 10:57:17.303: As1 IPCP: PrimaryDNS 0.0.0.0 (0x810600000000) Mar 13 10:57:17.303: As1 IPCP: PrimaryWINS 0.0.0.0 (0x820600000000) Mar 13 10:57:17.303: As1 IPCP: SecondaryDNS 0.0.0.0 (0x830600000000) Mar 13 10:57:17.303: As1 IPCP: SecondaryWINS 0.0.0.0 (0x840600000000) Mar 13 10:57:17.303: As1 IPCP: O CONFREJ [REQsent] id 1 len 22 Mar 13 10:57:17.303: As1 IPCP: CompressType VJ 15 slots CompressSlotID (0x0206002D0F01) Mar 13 10:57:17.303: As1 IPCP: PrimaryWINS 0.0.0.0 (0x820600000000) Mar 13 10:57:17.303: As1 IPCP: SecondaryWINS 0.0.0.0 (0x840600000000) Mar 13 10:57:17.319: As1 CCP: I CONFREQ [Not negotiated] id 1 len 15 Mar 13 10:57:17.319: As1 CCP: MS-PPC supported bits 0x00000001 (0x120600000001) Mar 13 10:57:17.319: As1 CCP: Stacker history 1 check mode EXTENDED (0x1105000104) Mar 13 10:57:17.319: As1 LCP: O PROTREJ [Open] id 3 len 21 protocol CCP Mar 13 10:57:17.319: As1 LCP: (0x80FD0101000F12060000000111050001) Mar 13 10:57:17.319: As1 LCP: (0x04) Mar 13 10:57:17.319: As1 IPCP: I CONFACK [REQsent] id 1 len 10 Mar 13 10:57:17.319: As1 IPCP: Address 172.22.66.23 (0x0306AC164217) Mar 13 10:57:18.191: %LINEPROTO-5-UPDOWN: Line protocol on Interface Async1, changed state to up Mar 13 10:57:19.191: As1 IPCP: TIMEout: State ACKrcvd Mar 13 10:57:19.191: As1 IPCP: O CONFREQ [ACKrcvd] id 2 len 10 Mar 13 10:57:19.191: As1 IPCP: Address 172.22.66.23 (0x0306AC164217) Mar 13 10:57:19.315: As1 IPCP: I CONFACK [REQsent] id 2 len 10 Mar 13 10:57:19.315: As1 IPCP: Address 172.22.66.23 (0x0306AC164217) Mar 13 10:57:20.307: As1 IPCP: I CONFREQ [ACKrcvd] id 2 len 34 Mar 13 10:57:20.307: As1 IPCP: Address 0.0.0.0 (0x030600000000) Mar 13 10:57:20.307: As1 IPCP: PrimaryDNS 0.0.0.0 (0x810600000000) Mar 13 10:57:20.307: As1 IPCP: PrimaryWINS 0.0.0.0 (0x820600000000) Mar 13 10:57:20.307: As1 IPCP: SecondaryDNS 0.0.0.0 (0x830600000000) Mar 13 10:57:20.307: As1 IPCP: SecondaryWINS 0.0.0.0 (0x840600000000) Mar 13 10:57:20.307: As1 IPCP: O CONFREJ [ACKrcvd] id 2 len 16 Mar 13 10:57:20.307: As1 IPCP: PrimaryWINS 0.0.0.0 (0x820600000000) Mar 13 10:57:20.307: As1 IPCP: SecondaryWINS 0.0.0.0 (0x840600000000) Mar 13 10:57:20.419: As1 IPCP: I CONFREQ [ACKrcvd] id 3 len 22 Mar 13 10:57:20.419: As1 IPCP: Address 0.0.0.0 (0x030600000000) Mar 13 10:57:20.419: As1 IPCP: PrimaryDNS 0.0.0.0 (0x810600000000) Mar 13 10:57:20.419: As1 IPCP: SecondaryDNS 0.0.0.0 (0x830600000000) Mar 13 10:57:20.419: As1 IPCP: O CONFNAK [ACKrcvd] id 3 len 22 Mar 13 10:57:20.419: As1 IPCP: Address 10.1.1.1 (0x03060A010101) Mar 13 10:57:20.419: As1 IPCP: PrimaryDNS 171.68.10.70 (0x8106AB440A46) Mar 13 10:57:20.419: As1 IPCP: SecondaryDNS 171.68.10.140 (0x8306AB440A8C) Mar 13 10:57:20.543: As1 IPCP: I CONFREQ [ACKrcvd] id 4 len 22 Mar 13 10:57:20.543: As1 IPCP: Address 10.1.1.1 (0x03060A010101) Mar 13 10:57:20.547: As1 IPCP: PrimaryDNS 171.68.10.70 (0x8106AB440A46) Mar 13 10:57:20.547: As1 IPCP: SecondaryDNS 171.68.10.140 (0x8306AB440A8C) Mar 13 10:57:20.547: As1 IPCP: O CONFACK [ACKrcvd] id 4 len 22 Mar 13 10:57:20.547: As1 IPCP: Address 10.1.1.1 (0x03060A010101) Mar 13 10:57:20.547: As1 IPCP: PrimaryDNS 171.68.10.70 (0x8106AB440A46) Mar 13 10:57:20.547: As1 IPCP: SecondaryDNS 171.68.10.140 (0x8306AB440A8C) Mar 13 10:57:20.547: As1 IPCP: State is Open Mar 13 10:57:20.551: As1 IPCP: Install route to 10.1.1.1
Note: Your debugs may appear in a different format. This example shows the newer PPP debugging output format which was modified in IOS version 11.2(8). See Chapter 16 for an example of PPP debugging with the older versions of IOS.
PPP LCP Negotiation Details
| Time Stamp | Description |
|---|---|
| 10:57:15.415 | Outgoing configuration request (O CONFREQ). The NAS sends an outgoing PPP configuration request packet to the client. |
| 10:57:15.543 | Incoming configuration acknowledgment (I CONFACK). The client acknowledges Montecito's PPP request. |
| 10:57:16.919 | Incoming configuration request (I CONFREQ). The client wants to negotiate the callback protocol. |
| 10:57:16.919 | Outgoing configuration reject (O CONFREJ). The NAS rejects the callback option. |
| 10:57:17.047 | Incoming configuration request (I CONFREQ). The client requests a new set of options. Notice that Microsoft Callback is not requested this time. |
| 10:57:17.047 | Outgoing configuration acknowledgment (O CONFACK). The NAS accepts the new set of options. |
| 10:57:17.047 | PPP LCP negotiation is completed successfully. The LCP state is "Open". Both sides have acknowledged (CONFACK) the other side's configuration request (CONFREQ). |
| 10:57:17.047 until 10:57:17.191 | PPP authentication is completed successfully. After the LCP negotiates, authentication starts. Authentication must take place before any network protocols, such as IP, are delivered. Both sides authenticate with the method negotiated during LCP. Montecito is authenticating the client using CHAP. |
| 10:57:20.551 | The state is open for IP Control Protocol (IPCP). A route is negotiated and installed for the IPCP peer, which is assigned IP address 1.1.1.1. |
Link Control Protocol
Two types of problems are typically encountered during LCP negotiation.
The first occurs when one peer makes configuration requests which the other peer cannot or will not acknowledge. While this is a frequent occurrence, it can be a problem if the requester insists on the parameter. A typical example is when negotiating AUTHTYPE (also known as "AuthProto"). For instance, many access servers are configured to accept only CHAP for authentication. If the caller is configured to do only PAP authentication, CONFREQs and CONFNAKs will be exchanged until one peer or the other drops the connection.
BR0:1 LCP: I CONFREQ [ACKrcvd] id 66 len 14 BR0:1 LCP: AuthProto PAP (0x0304C023) BR0:1 LCP: MagicNumber 0xBC6B9F91 (0x0506BC6B9F91) BR0:1 LCP: O CONFNAK [ACKrcvd] id 66 len 9 BR0:1 LCP: AuthProto CHAP (0x0305C22305) BR0:1 LCP: I CONFREQ [ACKrcvd] id 67 len 14 BR0:1 LCP: AuthProto PAP (0x0304C023) BR0:1 LCP: MagicNumber 0xBC6B9F91 (0x0506BC6B9F91) BR0:1 LCP: O CONFNAK [ACKrcvd] id 67 len 9 BR0:1 LCP: AuthProto CHAP (0x0305C22305) BR0:1 LCP: I CONFREQ [ACKrcvd] id 68 len 14 BR0:1 LCP: AuthProto PAP (0x0304C023) BR0:1 LCP: MagicNumber 0xBC6B9F91 (0x0506BC6B9F91) BR0:1 LCP: O CONFNAK [ACKrcvd] id 68 len 9 BR0:1 LCP: AuthProto CHAP (0x0305C22305) ... ...
The second type of problem in LCP is when only outbound CONFREQs are seen on one or both peers as in the example below. This is usually the result of what is referred to as a speed mismatch at the lower layer. This condition can occur in either async or ISDN DDR.
Jun 10 19:57:59.768: As5 PPP: Phase is ESTABLISHING, Active Open Jun 10 19:57:59.768: As5 LCP: O CONFREQ [Closed] id 64 len 25 Jun 10 19:57:59.768: As5 LCP: ACCM 0x000A0000 (0x0206000A0000) Jun 10 19:57:59.768: As5 LCP: AuthProto CHAP (0x0305C22305) Jun 10 19:57:59.768: As5 LCP: MagicNumber 0x5779D9D2 (0x05065779D9D2) Jun 10 19:57:59.768: As5 LCP: PFC (0x0702) Jun 10 19:57:59.768: As5 LCP: ACFC (0x0802) Jun 10 19:58:01.768: As5 LCP: TIMEout: State REQsent Jun 10 19:58:01.768: As5 LCP: O CONFREQ [REQsent] id 65 len 25 Jun 10 19:58:01.768: As5 LCP: ACCM 0x000A0000 (0x0206000A0000) Jun 10 19:58:01.768: As5 LCP: AuthProto CHAP (0x0305C22305) Jun 10 19:58:01.768: As5 LCP: MagicNumber 0x5779D9D2 (0x05065779D9D2) Jun 10 19:58:01.768: As5 LCP: PFC (0x0702) Jun 10 19:58:01.768: As5 LCP: ACFC (0x0802). Jun 10 19:58:03.768: As5 LCP: TIMEout: State REQsent Jun 10 19:58:03.768: As5 LCP: O CONFREQ [REQsent] id 66 len 25 Jun 10 19:58:03.768: As5 LCP: ACCM 0x000A0000 (0x0206000A0000) Jun 10 19:58:03.768: As5 LCP: AuthProto CHAP (0x0305C22305) Jun 10 19:58:03.768: As5 LCP: MagicNumber 0x5779D9D2 (0x05065779D9D2) Jun 10 19:58:03.768: As5 LCP: PFC (0x0702) Jun 10 19:58:03.768: As5 LCP: ACF.C (0x0802) Jun 10 19:58:05.768: As5 LCP: TIMEout: State REQsent Jun 10 19:58:05.768: As5 LCP: O CONFREQ [REQsent] id 67 len 25 !--- This repeats every two seconds until: Jun 10 19:58:19.768: As5 LCP: O CONFREQ [REQsent] id 74 len 25 Jun 10 19:58:19.768: As5 LCP: ACCM 0x000A0000 (0x0206000A0000) Jun 10 19:58:19.768: As5 LCP: AuthProto CHAP (0x0305C22305) Jun 10 19:58:19.768: As5 LCP: MagicNumber 0x5779D9D2 (0x05065779D9D2) Jun 10 19:58:19.768: As5 LCP: PFC (0x0702) Jun 10 19:58:19.768: As5 LCP: ACFC (0x0802) Jun 10 19:58:21.768: As5 LCP: TIMEout: State REQsent Jun 10 19:58:21.768: TTY5: Async Int reset: Dropping DTR
If the connection is async, the probable cause is a speed mismatch between the router and its modem. This is usually as a result of having failed to lock the DTE speed of the modem to the configured speed of the TTY line. The problem may be found on either or both of the peers, so check both. Refer to Modem Cannot Send or Receive Data earlier in this chapter.
If the symptoms are seen when the connection is over ISDN, the problem is likely to be that one peer is connecting at 56K while the other is at 64K. While this condition is rare, it does happen. The problem could be one or both peers, or possibly the telephone company. Use debug isdn q931 and examine the SETUP messages on each of the peers. The Bearer Capability sent from one peer should match the Bearer Capability seen in the SETUP message received on the other peer. As a possible remedy, configure the dialing speed, 56K or 64K, in either the interface level command dialer map or in the command dialer isdn speed configured under a map-class.
*Mar 20 21:07:45.033: ISDN BR0: TX -> SETUP pd = 8 callref = 0x2C *Mar 20 21:07:45.037: Bearer Capability i = 0x8890 *Mar 20 21:07:45.041: Channel ID i = 0x83 *Mar 20 21:07:45.041: Keypad Facility i = 0x35353533373539
This situation is one which may warrant a call to the Cisco TAC. Collect the following outputs from both peers before calling the TAC:
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show running-config
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show version
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debug isdn q931
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debug isdn events
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debug ppp negotiation
Authentication
Failed authentication is the single most common reason for a PPP failure. Misconfigured or mismatched usernames and passwords create error messages in debug output.
The following example shows that the username Goleta does not have permission to dial in to the NAS, which does not have a local username configured for this user. To fix the problem, use the username name password password command to add the username "Goleta" to the NAS' local AAA database:
Mar 13 11:01:42.399: As2 LCP: State is Open Mar 13 11:01:42.399: As2 PPP: Phase is AUTHENTICATING, by this end Mar 13 11:01:42.399: As2 CHAP: O CHALLENGE id 1 len 28 from "Montecito" Mar 13 11:01:42.539: As2 CHAP: I RESPONSE id 1 len 30 from "Goleta" Mar 13 11:01:42.539: As2 CHAP: Unable to validate Response. Username Goleta not found Mar 13 11:01:42.539: As2 CHAP: O FAILURE id 1 len 26 msg is "Authentication failure" Mar 13 11:01:42.539: As2 PPP: Phase is TERMINATING
The following example shows that the username "Goleta" is configured on the NAS. However, the password comparison failed. To fix this problem, use the username name password password command to specify the correct login password for Goleta:
Mar 13 11:04:06.843: As3 LCP: State is Open Mar 13 11:04:06.843: As3 PPP: Phase is AUTHENTICATING, by this end Mar 13 11:04:06.843: As3 CHAP: O CHALLENGE id 1 len 28 from "Montecito" Mar 13 11:04:06.987: As3 CHAP: I RESPONSE id 1 len 30 from "Goleta" Mar 13 11:04:06.987: As3 CHAP: O FAILURE id 1 len 25 msg is "MD/DES compare failed" Mar 13 11:04:06.987: As3 PPP: Phase is TERMINATING
For more information on PAP authentication refer to Configuring and Troubleshooting PPP Password Authentication Protocol (PAP).
Network Control Protocol
After the peers have successfully performed the required authentication, the negotiation moves into the NCP phase. If both peers are properly configured, the NCP negotiation might look like the following example which shows a client PC dialing into and negotiating with a NAS:
solvang# show debug Generic IP: IP peer address activity debugging is on PPP: PPP protocol negotiation debugging is on *Mar 1 21:35:04.186: As4 PPP: Phase is UP *Mar 1 21:35:04.190: As4 IPCP: O CONFREQ [Not negotiated] id 1 len 10 *Mar 1 21:35:04.194: As4 IPCP: Address 10.1.2.1 (0x03060A010201) *Mar 1 21:35:04.282: As4 IPCP: I CONFREQ [REQsent] id 1 len 28 *Mar 1 21:35:04.282: As4 IPCP: CompressType VJ 15 slots CompressSlotID (0x0206002D0F01) *Mar 1 21:35:04.286: As4 IPCP: Address 0.0.0.0 (0x030600000000) *Mar 1 21:35:04.290: As4 IPCP: PrimaryDNS 0.0.0.0 (0x810600000000) *Mar 1 21:35:04.298: As4 IPCP: SecondaryDNS 0.0.0.0 (0x830600000000) *Mar 1 21:35:04.306: As4 IPCP: O CONFREJ [REQsent] id 1 len 10 *Mar 1 21:35:04.310: As4 IPCP: CompressType VJ 15 slots CompressSlotID (0x0206002D0F01) *Mar 1 21:35:04.314: As4 CCP: I CONFREQ [Not negotiated] id 1 len 15 *Mar 1 21:35:04.318: As4 CCP: MS-PPC supported bits 0x00000001 (0x120600000001) *Mar 1 21:35:04.318: As4 CCP: Stacker history 1 check mode EXTENDED (0x1105000104) *Mar 1 21:35:04.322: As4 LCP: O PROTREJ [Open] id 3 len 21 protocol CCP *Mar 1 21:35:04.326: As4 LCP: (0x80FD0101000F12060000000111050001) *Mar 1 21:35:04.330: As4 LCP: (0x04) *Mar 1 21:35:04.334: As4 IPCP: I CONFACK [REQsent] id 1 len 10 *Mar 1 21:35:04.338: As4 IPCP: Address 10.1.2.1 (0x03060A010201) *Mar 1 21:35:05.186: %LINEPROTO-5-UPDOWN: Line protocol on Interface Async4, changed state to up *Mar 1 21:35:07.274: As4 IPCP: I CONFREQ [ACKrcvd] id 2 len 22 *Mar 1 21:35:07.278: As4 IPCP: Address 0.0.0.0 (0x030600000000) *Mar 1 21:35:07.282: As4 IPCP: PrimaryDNS 0.0.0.0 (0x810600000000) *Mar 1 21:35:07.286: As4 IPCP: SecondaryDNS 0.0.0.0 (0x830600000000) *Mar 1 21:35:07.294: As4 IPCP: O CONFNAK [ACKrcvd] id 2 len 22 *Mar 1 21:35:07.298: As4 IPCP: Address 10.1.2.2 (0x03060A010202) *Mar 1 21:35:07.302: As4 IPCP: PrimaryDNS 10.2.2.3 (0x81060A020203) *Mar 1 21:35:07.310: As4 IPCP: SecondaryDNS 10.2.3.1 (0x83060A020301) *Mar 1 21:35:07.426: As4 IPCP: I CONFREQ [ACKrcvd] id 3 len 22 *Mar 1 21:35:07.430: As4 IPCP: Address 10.1.2.2 (0x03060A010202) *Mar 1 21:35:07.434: As4 IPCP: PrimaryDNS 10.2.2.3 (0x81060A020203) *Mar 1 21:35:07.442: As4 IPCP: SecondaryDNS 10.2.3.1 (0x83060A020301) *Mar 1 21:35:07.446: ip_get_pool: As4: validate address = 10.1.2.2 *Mar 1 21:35:07.450: ip_get_pool: As4: using pool default *Mar 1 21:35:07.450: ip_get_pool: As4: returning address = 10.1.2.2 *Mar 1 21:35:07.454: set_ip_peer_addr: As4: address = 10.1.2.2 (3) is redundant *Mar 1 21:35:07.458: As4 IPCP: O CONFACK [ACKrcvd] id 3 len 22 *Mar 1 21:35:07.462: As4 IPCP: Address 10.1.2.2 (0x03060A010202) *Mar 1 21:35:07.466: As4 IPCP: PrimaryDNS 10.2.2.3 (0x81060A020203) *Mar 1 21:35:07.474: As4 IPCP: SecondaryDNS 10.2.3.1 (0x83060A020301) *Mar 1 21:35:07.478: As4 IPCP: State is Open *Mar 1 21:35:07.490: As4 IPCP: Install route to 10.1.2.2
PPP NCP Negotiation Details
| Time Stamp | Description |
|---|---|
| 21:35:04.190 | Outgoing configuration request (O CONFREQ). The NAS sends an outgoing PPP configuration request packet containing its IP address to the peer. |
| 21:35:04.282 | Incoming CONFREQ. The peer requests to do VJ header compression. It needs an IP address for itself, as well as addresses of the primary and secondary DNS servers. |
| 21:35:04.306 | Outbound Config-Reject (CONFREJ). VJ header compression is rejected. |
| 21:35:04.314 until 21:35:04.330 | The peer sends a request to do Compression Control Protocol; the entire protocol is rejected by the NAS by means of a PROTREJ message. The peer should not (and does not) attempt to retry CCP. |
| 21:35:04.334 | The peer acknowledges the IP address of the NAS with a CONFACK. |
| 21:35:07.274 | Incoming CONFREQ. The peer no longer requests to do VJ header compression, but still needs an IP address for itself, as well as addresses of the primary and secondary DNS servers. |
| 21:35:07.294 | The NAS sends a CONFNAK containing the address it wants the peer to use, and addresses of the primary and secondary DNS servers. |
| 21:35:07.426 | The peer sends the addresses back to the NAS; an attempt to confirm that the addresses were properly received. |
| 21:35:07.458 | The NAS acknowledges the addresses with a CONFACK. |
| 21:35:07.478 | Each side of the connection having issued a CONFACK, negotiation finishes. The command show interfaces Async4 on the NAS shows "IPCP: Open". |
| 21:35:07.490 | A host route to the remote peer is installed in the NAS' routing table. |
It is possible for the peers to simultaneously negotiate more than one Layer 3 protocol. It is not uncommon, for instance, to see IP and IPX being negotiated. It is also possible for one protocol to successfully negotiate while the other fails to do so.
Troubleshooting NCP
Any problems which occur during NCP negotiation can typically be traced to the configurations of the negotiating peers. If PPP negotiation fails during the NCP phase, refer to the following steps:
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Verify interface protocol configuration
Examine the output of the privileged exec command show running-config. Verify that the interface is configured to support the protocol you wish to run over the connection.
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Verify interface address
Confirm that the interface in question has an address configured. If using ip unnumbered [interface-name] or ipx ppp-client loopback [number], ensure that the referenced interface is configured with an address.
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Verify client address availability
If the NAS is supposed to issue an IP address to the caller, ensure that such an address is available. The IP address to be handed out to the caller can be obtained through one of the following methods:
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Configure locally on the interface. Check the interface configuration for the command peer default ip address a.b.c.d. In practice, this method should only be used on interfaces which accept connections from a single caller, such as on an async (not a group-async) interface.
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Address pool locally configured on the NAS. The interface should have the command peer default ip address pool [pool-name]. In addition, the pool must be defined at the system level with the command ip local pool [pool-name] [first-address] [last-address]. The range of addresses defined in the pool should be large enough to accommodate as many simultaneously-connected callers as the NAS is capable.
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DHCP server. The NAS interface must be configured with the command peer default ip address dhcp. Furthermore, the NAS must be configured to point to a DHCP server with the global configuration command ip dhcp-server [address].
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AAA. If using TACACS+ or RADIUS for authorization, the AAA server can be configured to hand a specific IP address to a given caller every time that caller connects. See Chapter 16 for more information.
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Verify server address configuration
To return the configured addresses of Domain Name Servers or Windows NT servers in response to BOOTP requests, ensure that the global-level commands async-bootp dns-server [address] and async-bootp nbns-server [address] are configured.
Note: While the command async-bootp subnet-mask [mask] can be configured on the NAS, the subnet mask will not be negotiated between the NAS and a PPP dial-in client PC. Due to the nature of point-to-point connections, the client automatically uses the IP address of the NAS (learned during IPCP negotiation) as the default gateway. The subnet mask is not needed in that point-to-point environment. The PC knows that if the destination address does not match the local address, the packet should be forwarded to the default gateway (NAS) which is always reached via the PPP link.
Before Calling the Cisco Systems TAC Team
Before calling the Cisco Systems Technical Assistance Center (TAC), make sure you have read through this chapter and completed the actions suggested for your system's problem.
Additionally, do the following and document the results so that we can better assist you:
For all problems, collect the output of show running-config and show version. Ensure that the command service timestamps debug datetime msec is in the configuration.
For DDR problems, collect the following:
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show dialer map
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debug dialer
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debug ppp negotiation
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debug ppp authentication
If ISDN is involved, collect:
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show isdn status
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debug isdn q931
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debug isdn events
If modems are involved, collect:
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show lines
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show line [x]
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show modem (if integrated modems are involved)
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show modem version (if integrated modems are involved)
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debug modem
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debug modem csm (if integrated modems are involved)
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debug chat (if a DDR scenario)
If T1s or PRIs are involved, collect:
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show controller t1
Related Information
Revision History
| Revision | Publish Date | Comments |
|---|---|---|
1.0 |
14-Sep-2005 |
Initial Release |
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