Cisco CCNP Certification Exam Training:

Configuing ISDN Caller ID Callback On Cisco Routers

By Chris Bryant, CCIE #12933

You might have thought you were done with ISDN in your CCNA studies, but that was only the beginning! To earn your CCNP certification, you'll need to know how to configure, troubleshoot, and verify various ISDN features, and one of those is Caller ID Callback.

This feature is often referred to simply as "Caller ID", which can be a little misleading if you've never seen this service in operation before. To most of us, Caller ID is a phone service that displays the source phone number of an incoming call.  Caller ID Screening has a different meaning, though.  Caller ID Screening on a Cisco router is really another kind of password - it defines the phone numbers that are allowed to call the router. 

The list of acceptable source phone numbers is created with the isdn caller command.  Luckily for us, this command allows the use of x to specify a wildcard number.  The command isdn caller 555xxxx results in calls being accepted from any 7-digit phone number beginning with 555, and rejected in all other cases.  We'll 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 dialer will be run on R1 before sending the ping.

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 to abort.
Sending 5, 100-byte ICMP Echos to 172.12.12.2, timeout is 2 seconds:

03:30:25: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:30:25: BR0 DDR: Attempting to dial 8358662.
03:30:26: BRI0: wait for isdn carrier timeout, call id=0x8015
03:30:27: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:30:27: BR0 DDR: Attempting to dial 8358662.
03:30:28: BRI0: wait for isdn carrier timeout, call id=0x8016
03:30:29: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:30:29: BR0 DDR: Attempting to dial 8358662.
03:30:30: BRI0: wait for isdn carrier timeout, call id=0x8017
03:30:31: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:30:31: BR0 DDR: Attempting to dial 8358662.
03:30:32: BRI0: wait for isdn carrier timeout, call id=0x8018
03:30:33: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:30:33: BR0 DDR: Attempting to dial 8358662.
Success rate is 0 percent (0/5)
R1#
03:30:34: BRI0: wait for isdn carrier timeout, call id=0x8019
R1#

R1 doesn't give us any hints as to what the problem is, but we can see that the pings definitely aren't going through.  On R2, show dialer displays the number of screened calls.

R2#show dialer

BRI0 - dialer type = ISDN

Dial String      Successes   Failures    Last DNIS   Last status
8358661                  1          0    00:03:16       successful
7 incoming call(s) have been screened.
0 incoming call(s) rejected for callback.

The callback option mentioned in the last line shown above enables the router to reject a phone call, and then call that router back seconds later.

R2 will now be configured to initially hang up on R1, and then call R1 back. 

R2(config-if)#isdn caller 8358661 callback

R1 will now ping R2.  The pings aren't returned, but seconds later R2 calls R1 back.

R1#ping 172.12.12.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.12.2, timeout is 2 seconds:

03:48:03: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:48:03: BR0 DDR: Attempting to dial 8358662.
03:48:04: BRI0: wait for isdn carrier timeout, call id=0x801F
03:48:05: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:48:05: BR0 DDR: Attempting to dial 8358662.
03:48:06: BRI0: wait for isdn carrier timeout, call id=0x8020
03:48:07: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:48:07: BR0 DDR: Attempting to dial 8358662.
03:48:08: BRI0: wait for isdn carrier timeout, call id=0x8021
03:48:09: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:48:09: BR0 DDR: Attempting to dial 8358662.
03:48:10: BRI0: wait for isdn carrier timeout, call id=0x8022
03:48:11: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)
03:48:11: BR0 DDR: Attempting to dial 8358662.
Success rate is 0 percent (0/5)
R1#
03:48:12: BRI0: wait for isdn carrier timeout, call id=0x8023
R1#
03:48:18: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up
R1#
03:48:18: BR0:1 DDR: dialer protocol up
R1#
03:48:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state
to up
R1#
03:48:24: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 8358662 R2

show dialer on R2 shows the reason for the call to R1 is a callback return call.

R2#show dialer

BRI0 - dialer type = ISDN

Dial String      Successes   Failures    Last DNIS   Last status
8358661                  3          0    00:00:48       successful
7 incoming call(s) have been screened.
10 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 71 secs
Connected to 8358661 (R1)

The drawback with Caller ID Callback is that many telco switches do not support this feature, and that includes many ISDN simulators found in today's CCNA and CCNP home labs. Cisco documentation mentions this as well. PPP Callback is a better choice than Caller ID Callback overall, but in Ciscoland, it's always a good idea to know more than one way to do something - whether it's on the CCNP certification exams or in the network control room!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com