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 CCIEinterface 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 ChapMost 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 CCIEinterface 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 CALLR2BACK Broadcast 5558888 Dialer-group 1 Isdn Switch-type Basic-ni Ppp Callback Accept Ppp Authentication Chapmap-class Dialer CALLR2BACK Dialer Callback-server UsernameExamining 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 CALLR2BACK, 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.R2ping 172.12.12.1Type 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 (s172.12.12.2, D172.12.12.1)02:45:42: BR0 DDR: Attempting To Dial 555777702:45:42: LINK-3-UPDOWN: Interface BRI0:1, Changed State To Up02:45:42: BR0:1 DDR: Callback Negotiated - Disconnecting Now02:45:42: BR0:1 DDR: Disconnecting Call02:45:42: ISDN-6-CONNECT: Interface BRI0:1 Is Now Connected To 5557777 R102:45:42: LINK-3-UPDOWN: Interface BRI0:1, Changed State To Down02:45:42: DDR: Callback Client For R1 5557777 Created02:45:42: BR0:1 DDR: Disconnecting Call.....Success Rate Is 0 Percent (05)R202:45:57: LINK-3-UPDOWN: Interface BRI0:1, Changed State To UpR202:45:57: BR0:1 DDR: Callback Received From R1 555777702:45:57: DDR: Freeing Callback To R1 555777702:45:57: BR0:1 DDR: Dialer Protocol Up02:45:58: LINEPROTO-5-UPDOWN: Line Protocol On Interface BRI0:1, Changed State To UpThe 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.R1show DialerBRI0 - Dialer Type ISDNDial String Successes Failures Last DNIS Last Status5558888 2 4 00:00:20 Successful0 Incoming Call(s) Have Been Screened.0 Incoming Call(s) Rejected For Callback.BRI0:1 - Dialer Type ISDNIdle Timer (120 Secs), Fast Idle Timer (20 Secs)Wait For Carrier (30 Secs), Re-enable (15 Secs)Dialer State Is Data Link Layer UpDial Reason: Callback Return CallTime Until Disconnect 99 SecsConnected To 5558888 (R2)Pretty Cool! PPP Callback Isnt Just Important For Passing Your CCNA And CCNP Exams In Circumstances Such As Shown In This Example, It Can Save Your Organization Quite A Bit Of Money!
