Configuring MS Callback Between a Router and a Windows PC

Document ID: 9554

Introduction

The Microsoft implementation of callback is not compliant with RFC 1570 . However, because of the large market share of the Microsoft dial-up networking client, Cisco has implemented the Microsoft Callback (MSCB) Control Protocol into Cisco IOS® Software Release 11.3(2)T and later.

Requirements

Before you attempt this configuration, ensure that you meet these requirements:

• Configure the Network Access Server (NAS) to accept analog calls from the client. Callback is an additional feature of modem dial-in. Therefore, verify whether this aspect functions correctly. This can help you to troubleshoot.


• The T1/E1 circuit must be capable of dialout. Contact your telephone company (Telco) to verify this

Components Used

The information in this document is based on Cisco IOS Software Release 11.3(2)T and later versions. This scenario was tested on a PC with Windows dial-up networking.

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, see the Cisco Technical Tips Conventions.

Background Theory

Callback executes in this order:

1. A PC user (client) connects to the Cisco access server.
2. Callback process is negotiated in the Point-to-Point Protocol (PPP) link control protocol (LCP) phase.


3. PPP authentication is performed.


4. The Cisco IOS software validates callback rules for this user or line and disconnects the caller for callback.


5. The Cisco access server dials the client.

There are four types of MSCB:

1. No callback.


2. User-specified callback number.


3. Server-specified (preconfigured) callback number.


4. List of preconfigured callback number.

The default configuration is no callback (option 1). Options 2 or 3 can be configured:

• Locally (if no AAA server is used).


• In the TACACS+ or RADIUS user profile (if AAA is used).

If option 2 is configured, the user is prompted to enter his callback number. If option 3 is configured, the prompt only offers one choice, which is the administrator-defined number.

Cisco only implements the callback server functionality of MSCB and not the callback client functionality. This means that a Cisco router can be used only as an MSCB server and not as an MSCB client. In addition, the Cisco implementation of MSCB requires authentication to be performed on the client.

In this section, you are presented with the information to configure the features described in this document.

Configuration Summary

To enable MSCB, you must enable the ppp callback accept command under the receiving interface (for example, group-async). In addition, because authentication is required, you must enable Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP) authentication:   ppp authentication chap pap

Two chat-scripts are created automatically. These are the offhook and callback chat-scripts:

  chat-script offhook "" "ATH1" OK

  chat-script callback ABORT ERROR ABORT BUSY ""
  "ATZ" OK "ATDT \T" TIMEOUT60 CONNECT \c

  The chat-scripts are also automatically applied to the lines in use:
  line 1 24
  script modem-off-hook offhook

A user must be authorized to be called back. You can configure this locally on NAS or on the external AAA server (RADIUS or TACACS+), based on where the username and password information is stored.

  This is a local configuration for a user who is called back at 5551212:
  username callmeback callback-dialstring 5551212 password cisco
  This local configuration is applicable to users who are allowed to specify their own callback number:

  username callmeback callback-dialstring "" password cisco

Network Diagram

This document uses this network setup:

Configurations

This document uses this configuration:

• isdn2-2 (AS5200 Router)

Windows Client Configuration

Windows 95 and 98 Client Configuration

For Windows 95 and 98 PCs, there is no special client-side configuration for callback. The access server handles the callback features of the connection. The Windows 95 or 98 PC displays a "waiting for callback" message to indicate that a callback is in progress.

Windows NT and 2000 Client Configuration

Configure these platforms to request callback. Complete these steps to configure them:

1. hoose Start > Programs > Accessories > Communications > Network and Dial-up Connections.


2. hoose Advanced > Dial-up Preferencesfrom the menu.


3. Click the Callback tab to access the callback features menu as shown in figure 1.


4. Configure your callback options as needed:


• In order not to use the callback function, click the No Callback button.



• To be prompted what to do when a server offers callback, click the Ask Me During Dialing When The Server Offers button.


¨
• To automatically accept callback offers, click the Always Call Me Back At The Number(s) Below button, and select the device to use from the list.


¨
• To change the callback phone number, select the device and click the Edit button. Enter the number in the Phone Number field as shown in Figure 1, and then click OK in the Call Me Back At dialog box.


5. Click the Phone Number field, and enter the number in the Call Me Back At dialog box (shown in figure 1). Click OK when you are finished.


6. When you are finished, click OK in the Dial-up Preferences dialog box.

Figure 1  Access the Callback Features

Verify

This section provides information you can use to confirm that your configuration is working properly.

Certain showcommands are supported by the Output Interpreter Tool ( registered customers only) , which allows you to view an analysis of show command output.

Troubleshoot

This section provides information you can use to troubleshoot your configuration.

Troubleshooting Commands

Note: Before issuing debug commands, please see Important Information on Debug Commands.

For more information on debug commands, see the Cisco IOS Release 12.0 Debug Command Reference.

• debug aaa authenticationdisplays information on AAA authentication.


• debug aaa authorizationdisplays information on AAA authorization.


• debug callbackdisplays callback events when the router uses a modem and a chat script to call back on a terminal line.


• debug modemenables you to observe modem line activity on an access server.


• debug ppp [ packet | negotiation | error | authentication ] displays information on traffic and exchanges in an internetwork that implements PPP.

• debug chatshows the handshake that occurs between the access server and its internal modem while the modem is instructed to dial out. A chat-script is a set of expect-send string pairs that define the handshake between data terminal equipment (DTE) and data communications equipment (DCE) devices.


• debug isdn q931displays the ISDN Q.931 (D channel) call setup and teardown messages and debugs. In this scenario, the modem call is carried as a voice bearer service over the Public Switched Telephone Network (PSTN).


• debug modem csmenables you to troubleshoot Call Switching Module (CSM) problems on routers with internal digital modems. With this command, you can trace the complete sequence of switching incoming and outgoing calls.

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Related Information

• Configuring Asynchronous Callback


• PPP Callback over ISDN


• Configuring PPP Callback for DDR


• Configuring PPP Callback with TACACS+


• Configuring PPP Callback with RADIUS


• Access Products Support Pages


• Dial Technology Support Pages


• Technical Support - Cisco Systems

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