3.3 PPP Callback
3.3.4 Configuring the callback server
A router can call back PPP clients that dial in. As shown in the Figure, to configure PPP callback for a server, perform the following steps (the following step numbers correspond to the numbers in the Figure):
  1. Configure IP on the dial-in line.
  2. Use the dialer callback-secure command to disconnect calls that are not properly configured for callback. A callback server with dialer callback-secure configured disconnects any unconfigured dial-in users.
  3. Set up dialer map with the dialer map and dialer-group commands.
  4. Use the ppp callback accept command to enable callback.
  5. Define the PPP authentication method with the ppp authentication chap command.
  6. Configure dialer callback-server username in a dialer map-class to identify the name in the dialer map as a valid callback client. When callback client router dials in and is authenticated, the call is disconnected. For example, in the Figure, a return call is made to 555-5678, as configured by the dialer map command. The dialer map command identifies the map class to be used for this connection.
  7. Use the dialer hold-queue timeout command to set the server hold-queue timer, which identifies the number of seconds that a callback client or server holds packets destined for the client while waiting for the return call to be completed.

Use the dialer enable-timeout command to determine the amount of time that the server waits before making the return call.

PPP Callback-Additional Feature from Cisco

Another command for callback is dialer callback-secure, which is a Cisco proprietary command. This command ensures that the initial call is always disconnected at the receiving end and that the return call is made only if the username is configured for callback. If the username (hostname in the dialer map  command) is not configured for callback, the initial call stays up and no return call is made.