Enabling PPP Callback over a Dialer/ISDN Interface

Definitions:

• Client: router making the intial call to the server.
• Server: router accepting the intial call and making the return call to the client.

Actions:

1. Configure client and server for PPP with PAP or CHAP authentication and DDR.

2. Verify connectivity from client to server without callback using DDR.

3. Client configuration:

   Enter ppp callback request at an interface configuration prompt on the client router. You can use ? for help. Callback will be requested calls going out that interface.

   Make sure there is a dialer map command for the server which includes the server's hostname.

4. Server configuration:

   Enter map-class dialer <name> at the global configuration prompt. You are now in map-class configuration.

   Enter dialer callback-server username and then exit.

   Enter ppp callback accept at an interface configuration prompt on the server router. You can use ? for help. Callback will be accepted for all calls coming into that interface which request callback.

   Add the string class <name> to the dialer map command before the dial string which defines the clients phone number and IP address. Make sure the clients hostname is a part of the same dialer map command.

   Using dialer callback-secure at the config interface prompt will make the callback server always disconnect the initial call and only callback if the server has a complete config for the remote end. (i.e. dialer map with class and class defined).

   The default is no dialer callback-secure which will keep up the initial call if the server does not have a complete config for the remote.

5. At this point, callback should be enabled. When using DDR to access the server from the client, you should see the BRI line go up, then down very quickly, and then up again.

Notes:

• You should be able to use the ? help for any of the commands.

• The enable timeout of the callback server determines how long to wait after disconnecting the intial call before making the return call. Configure dialer enable 1 to callback within one second.

• Hold queues have been extended when callback is configured. Both the client and server can have hold queues which will hold packets destined for the remote destination until the return call is connected or the holdq timer expires. There is a config option for the dialer hold-queue command which allows configuration of the hold queue timer.

  The time entered in the timeout option of the dialer hold-queue command is added to the enable time for the same interface. The sum is used to determine the actual timeout.

       Command:  dialer holdq <0-xx> [timeout <1-xx>] 
                 The default for the timeout option is 45 sec.
  

• Debugs:

        debug dialer  (most important debug)
        debug ppp negotiation
        debug ppp chap
  

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