Cisco CCNP Certification Exam Training:

Configuring PPP Callback On A Cisco Router

By Chris Bryant, CCIE #12933

You learned about PPP in your CCNA studies, but earning your CCNP certification means taking your PPP knowledge to the next level. Let's take a look at PPP callback and how to configure this feature on a Cisco router.

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)

To earn your CCNP certification, it's not quite enough to know how to configure a Cisco router feature - you've got to know how to verify and troubleshoot that feature as well. Don't forget about your show and debug commands!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com