Configuring Point-to-Point Callback for Dial-on-Demand Routing

This chapter describes how to configure PPP callback for dial-on-demand routing (DDR). It includes the following main sections:

This feature implements the following callback specifications of RFC 1570:

For the client---Option 0, location is determined by user authentication.
For the server---Option 0, location is determined by user authentication; Option 1, dialing string; and Option 3, E.164 number.

Return calls are made through the same dialer rotary group but not necessarily the same line as the initial call.

For a complete description of the PPP callback commands in this chapter, see the Cisco IOS Dial Services Command Reference publication. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.

PPP Callback for DDR Overview

PPP callback provides a client/server relationship between the endpoints of a point-to-point connection. PPP callback allows a router to request that a dialup peer router call back. The callback feature can be used to control access and toll costs between the routers.

When PPP callback is configured on the participating routers, the calling router (the callback client) passes authentication information to the remote router (the callback server), which uses the host name and dial string authentication information to determine whether to place a return call. If the authentication is successful, the callback server disconnects and then places a return call. The remote username of the return call is used to associate it with the initial call so that packets can be sent.

Both routers on a point-to-point link must be configured for PPP callback; one must function as a callback client and one must be configured as a callback server. The callback client must be configured to initiate PPP callback requests, and the callback server must be configured to accept PPP callback requests and place return calls.

See the section "MS Callback Overview" later in this chapter if you are using PPP callback between a Cisco router or access server and client devices configured for Windows 95 and Windows NT.

Note If the return call fails (because the line is not answered or the line is busy), no retry occurs. If the callback server has no interface available when attempting the return call, it does not retry.

PPP Callback for DDR Configuration Task List

To configure PPP callback for DDR, perform the following tasks:

Configuring a Router as a Callback Client (Required)
Configuring a Router as a Callback Server (Required)

For an example of configuring PPP callback, see the section "PPP Callback Examples" at the end of this chapter.

Configuring a Router as a Callback Client

To configure a router interface as a callback client, use the following commands beginning in global configuration mode:

Command Purpose

Step 1

interface type number
Specifies the interface.

Step2

dialer in-band [no-parity | odd-parity]
Enables DDR. Specifies parity, if needed, on synchronous or asynchronous serial interfaces.

Step3

encapsulation ppp
Enables PPP encapsulation.

Step4

ppp authentication chap
or

ppp authentication pap
Enables Challenge Handshake Authentication Protocol (CHAP) or Password Authentication Protocol (PAP) authentication.

Step5

dialer map protocol next-hop-address name hostname dial-string
Maps the next hop address to the host name and phone number.

Step6

ppp callback request
Enables the interface to request PPP callback for this callback map class.

Step7

dialer hold-queue packets timeout seconds
(Optional) Configures a dialer hold queue to store packets for this callback map class.

	Command	Purpose
Step 1	interface type number	Specifies the interface.
Step2	dialer in-band [no-parity \| odd-parity]	Enables DDR. Specifies parity, if needed, on synchronous or asynchronous serial interfaces.
Step3	encapsulation ppp	Enables PPP encapsulation.
Step4	ppp authentication chap or ppp authentication pap	Enables Challenge Handshake Authentication Protocol (CHAP) or Password Authentication Protocol (PAP) authentication.
Step5	dialer map protocol next-hop-address name hostname dial-string	Maps the next hop address to the host name and phone number.
Step6	ppp callback request	Enables the interface to request PPP callback for this callback map class.
Step7	dialer hold-queue packets timeout seconds	(Optional) Configures a dialer hold queue to store packets for this callback map class.

Configuring a Router as a Callback Server

To configure a router as a callback server, use the following commands beginning in global configuration mode:

Command Purpose

Step1

interface type number
Specifies the interface and enters interface configuration mode.

Step2

dialer in-band [no-parity | odd-parity]
Enables DDR. Specifies parity, if needed, on synchronous or asynchronous serial interfaces.

Step3

encapsulation ppp
Enables PPP encapsulation.

Step4

ppp authentication {chap | pap}
Enables CHAP or PAP authentication.

Step5

dialer map protocol next-hop-address name hostname class classname dial-string
Maps the next hop address to the host name and phone number, using the name of the map class established for PPP callback on this interface.

Step6

dialer hold-queue number timeout seconds
(Optional) Configures a dialer hold queue to store packets to be transferred when the callback connection is established.

Step7

dialer enable-timeout seconds
(Optional) Configures a timeout period between calls.

Step8

ppp callback accept
Configures the interface to accept PPP callback.

Step9

isdn fast-rollover-delay seconds
(ISDN only) Configures the time to wait before another call is placed on a B channel, to allow the prior call to be torn down completely.

Step10

dialer callback-secure
(Optional) Enables callback security, if desired.

Step11

exit
Returns to global configuration mode.

Step12

map-class dialer classname
Configures a dialer map class for PPP callback.

Step13

dialer callback-server [username]
Configures a dialer map class as a callback server.

	Command	Purpose
Step1	interface type number	Specifies the interface and enters interface configuration mode.
Step2	dialer in-band [no-parity \| odd-parity]	Enables DDR. Specifies parity, if needed, on synchronous or asynchronous serial interfaces.
Step3	encapsulation ppp	Enables PPP encapsulation.
Step4	ppp authentication {chap \| pap}	Enables CHAP or PAP authentication.
Step5	dialer map protocol next-hop-address name hostname class classname dial-string	Maps the next hop address to the host name and phone number, using the name of the map class established for PPP callback on this interface.
Step6	dialer hold-queue number timeout seconds	(Optional) Configures a dialer hold queue to store packets to be transferred when the callback connection is established.
Step7	dialer enable-timeout seconds	(Optional) Configures a timeout period between calls.
Step8	ppp callback accept	Configures the interface to accept PPP callback.
Step9	isdn fast-rollover-delay seconds	(ISDN only) Configures the time to wait before another call is placed on a B channel, to allow the prior call to be torn down completely.
Step10	dialer callback-secure	(Optional) Enables callback security, if desired.
Step11	exit	Returns to global configuration mode.
Step12	map-class dialer classname	Configures a dialer map class for PPP callback.
Step13	dialer callback-server [username]	Configures a dialer map class as a callback server.

NoteOn the PPP callback server, the dialer enable-timeout functions as the timer for returning calls to the callback client.

MS Callback Overview

MS Callback provides client/server callback services for Microsoft Windows 95 and Microsoft Windows NT clients. MS Callback supports the Microsoft Callback Control Protocol (MSCB). MSCB is a Microsoft proprietary protocol that is used by Windows 95 and Windows NT clients. MS Callback supports negotiated PPP Link Control Protocol (LCP) extensions initiated and agreed upon by the Microsoft client. The MS Callback feature is added to existing PPP Callback functionality. Therefore, if you configure your Cisco access server to perform PPP Callback using Cisco IOS Release 11.3(2)T or later, MS Callback is automatically available.

MS Callback supports AAA security models using a local database or AAA server.

MSCB uses LCP callback options with suboption type 6 . The Cisco MS Callback feature supports clients with a user-specified callback number and server specified (preconfigured) callback number.

MS Callback does not affect non-Microsoft machines that implement standard PPP LCP extensions as described in RFC 1570. In this scenario, MS Callback is transparent.

The following are restrictions of the MS Callback feature:

The Cisco access server and client must be configured for PPP and PPP callback.
The router or access server must be configured to use CHAP or PAP authorization.
MS Callback is only supported on the Public Switched Telephone Network (PSTN) and ISDN links.
MS Callback is only supported for IP.

Configuring MS Callback

If you configure the Cisco access server for PPP callback, MSCallback is enabled by default. You need not configure additional parameters on the Cisco access server. To debug PPP connections using MS Callback, see the debug ppp cbcp command in the Cisco IOS Debug Command Reference publication.

PPP Callback Examples

The following example configures a PPP callback server and client to call each other.

The PPP callback server is configured on an ISDN BRI interface in a router in Atlanta. The callback server requires an enable timeout and a map class to be defined.

The PPP callback client is configured on an ISDN BRI interface in a router in Dallas. The callback client does not require an enable timeout and a map class to be defined.

PPP Callback Server

interface bri 0
 ip address 7.1.1.7 255.255.255.0
 encapsulation ppp
 dialer callback-secure
 dialer enable-timeout 2
 dialer map ip 7.1.1.8 name atlanta class dial1 81012345678901
 dialer-group 1
 ppp callback accept
 ppp authentication chap
!
map-class dialer dial1
 dialer callback-server username

PPP Callback Client

interface bri 0
 ip address 7.1.1.8 255.255.255.0
 encapsulation ppp
 dialer map ip 7.1.1.7 name dallas 81012345678902
 dialer-group 1
 ppp callback request
 ppp authentication chap

Table of Contents

Configuring Point-to-Point Callback for Dial-on-Demand Routing