Cisco CCNA / CCNP Certification Exam: Caller ID Screening And Callback

As a CCNA and / or CCNP candidates, which you have to be on the ground in situations where Cisco router functions can be your customers money and time. For example, if a router said calls for a hub router, and the tolls on the website said are higher than those of the hub router, the hub router depends first and then call back saying the router can save the client money (and you look well!)
A popular way of doing this is using PPP callback, but as we all know, it is a good idea to know more than one way to do things and Cisco world! A less well-known but effective method of screening callback is Caller ID & recall. Before the callback function, but we need to know what Caller ID screening in the first place!
This function is often referred to simply as " " Caller ID, which is a little misleading if you have never seen service and operations. For most of us, Caller ID is a telephone service shows that the source for the telephone number of an incoming call. Caller ID screening has a different meaning, though. Caller ID screening of a Cisco router is really a different kind of password - it defines the telephone numbers that are allowed to view the list of permissible router.
The source phone numbers, with the command isdn caller. Fortunately for us, this command allows the use of x is a wild card. The command isdn caller 555xxxx results and calls will be accepted by all 7-digit number beginning with 555, rejected and in all other cases. We configure R2 to do just that and then send a ping from R1 to R2. To see the results of the Caller ID screening, debug dialers are R1 before sending the ping. I have handled this issue, as the output you see here is repeated fires times - once for each ping packet.
R2 (config-if) isdn caller 555xxxx
R1 debug dialer
Dial on-demand events debugging is on
R1 ping 172.12.12.2
Type escape sequence abort.
Sending 5, 100-byte ICMP echo to 172.12.12.2, 2 second timeout:
03: 30:25: BR0 DDR: Dialing ip cause (n = 172.12.12.1, d = 172.12.12.2)
03 : 30:25: BR0 DDR: Attempting to dial 8358662.
Success is 0 percent (0 / 5)
R1 gives us no indication as to what the problem is, but we can see that the pings are definitely not. On R2, show dialer shows the number of screened calls.
R2 show dialer
BRI0 - dialer type = string ISDN
Dial successes Failures Last DNIS Last status
8358661 1 0 00:03:16 successful
7 incoming call (s) screened.
0 incoming call (s) Rejected callback.
The callback option, in the last line shown above enables the router to reject a call, and then call back that second later.
R2 router will now be configured to hang on to first R1, and then back R1.
R2 (config-if) 8358661 isdn caller callback
R1 now ping R2. The pings are not returned, but seconds later R2 calls R1 back.
R1 ping 172.12.12.2
Success is 0 percent (0 / 5)
R1
03: 48:12: BRI0: wait isdn carrier timeout, call id = 0x8023
R1
03 : 48:18:% LINK-3-UPDOWN: Interface BRI0: 1, as amended state up
R1
03: 48:18: BR0: 1 GDR: Dialer protocol up
R1
03: 48:19:% LINEPROTO-5 - UPDOWN: Line Interface Protocol on BRI0: 1, as amended state up
R1
03: 48:24:% ISDN 6-CONNECT: Interface BRI0: 1 is now with 8358662 R2
show dialer on R2 shows that the reason for the invocation of R1 callback is a return call.
R2 show dialer
BRI0 - dialer type = string ISDN
Dial successes Failures Last DNIS Last status
8358661 3 0 00:00:48 successful
7 incoming call (s) screened.
10 incoming call (s) rejected callback.
BRI0: 1 - dialer type = ISDN
Idle timer (120 seconds), Fast idle timer (20 sec)
Wait for carrier selection (30 sec), re-enable (15 sec)
Dialer State Data Link Layer up
Dial Reason: Callback call
Time return to 71 separate on Secs
Connected 8358661 (R1)
The disadvantage Caller ID Callback is that not all Telco switches, so that if you have the choice between this and PPP call back, you are probably better with PPP callback. But it is always a good idea to know more than just a way to take care of things with Cisco!
Chris Bryant, CCIE 12933, is the owner of The Bryant Advantage, home of free CCNA and CCNP tutorials, The Ultimate CCNA Study Package, Ultimate CCNP Study Packages.
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