Cisco CCNA - CCNP Certification Exam Tutorial: Configuring PPP Callback

You may run into situations where a router in a remote location needs to dial in to a central router, but the toll charges are much higher if the remote router makes the call. This scenario is perfect for PPP Callback, where the callback client places a call to a callback server, authentication takes place, and the server then hangs up on the client! This ensures that the client isn't charged for the call. The server then calls the client back.


In the following example, R2 has been configured as the client and R1 is the callback server. Let's look at both configurations and the unique commands PPP Callback requires.

Client:

username R1 password CCIE

interface BRI0

ip address 172.12.12.2 255.255.255.0

encapsulation ppp

dialer map ip 172.12.12.1 name R1 broadcast 5557777

dialer-group 1

isdn switch-type basic-ni

ppp callback request

ppp authentication chap

Most of that configuration will look familiar to you, but the ppp callback request command might not. This command enables the BRI interface to request the callback.

Simple enough, right? The PPP Callback Server config requires more configuration and an additional map-class as well.

Server:

username R2 password CCIE
interface BRI0

ip address 172.12.12.1 255.255.255.0

encapsulation ppp

dialer callback-secure

dialer map ip 172.12.12.2 name R2 class CALL_R2_BACK broadcast 5558888

dialer-group 1

isdn switch-type basic-ni

ppp callback accept

ppp authentication chap

map-class dialer CALL_R2_BACK

dialer callback-server username


Examining the PPP Callback Server command from the top down...


dialer callback-secure enables security on the callback. If the remote router cannot be authenticated for callback, the incoming call will be disconnected.


The dialer map statement now calls the class CALL_R2_BACK, shown at the bottom of the config excerpt.


ppp callback accept enables PPP callback on this router.


dialer callback-server username tells the callback server that the device referenced in the dialer map statement is a callback client.