Configuring PPP Callback

By: Chris Bryant

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.

The only way to find out if the config works is to test it, so let's send a ping from R2 to R1 and see if the callback takes place.

R2#ping 172.12.12.1

Type escape sequence to abort.


Sending 5, 100-byte ICMP Echos to 172.12.12.1, timeout is 2 seconds:

02:45:42: BR0 DDR: Dialing cause ip (s=172.12.12.2, d=172.12.12.1)


02:45:42: BR0 DDR: Attempting to dial 5557777


02:45:42: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up


02:45:42: BR0:1 DDR: Callback negotiated - Disconnecting now


02:45:42: BR0:1 DDR: disconnecting call


02:45:42: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 5557777 R1


02:45:42: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down


02:45:42: DDR: Callback client for R1 5557777 created


02:45:42: BR0:1 DDR: disconnecting call.....


Success rate is 0 percent (0/5)


R2#


02:45:57: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up


R2#


02:45:57: BR0:1 DDR: Callback received from R1 5557777


02:45:57: DDR: Freeing callback to R1 5557777


02:45:57: BR0:1 DDR: dialer protocol up


02:45:58: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to up

The callback was successfully negotiated, and the call then disconnected.  R1 then called R2 back, and show dialer on R1 confirms the purpose of the call.

R1#show dialer

BRI0 - dialer type = ISDN

Dial String      Successes   Failures    Last DNIS   Last status


5558888                  2          4    00:00:20       successful


0 incoming call(s) have been screened.


0 incoming call(s) rejected for callback.

BRI0:1 - dialer type = ISDN


Idle timer (120 secs), Fast idle timer (20 secs)


Wait for carrier (30 secs), Re-enable (15 secs)


Dialer state is data link layer up


Dial reason: Callback return call


Time until disconnect 99 secs


Connected to 5558888 (R2)

Pretty cool!  PPP Callback isn’t just important for passing your CCNA and CCNP exams – in circumstances such as shown in this exampleArticle Submission, it can save your organization quite a bit of money!

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