The success of the CCNA examination relies upon the mastery of many new technologies and few subjects have more detailed information than ISDN. However, ISDN does not apply only to your CCNA exams. While ISDN is rejected by many, many small and medium sized networks use ISDN for their frame relay backup. Some of these organisations use ISDN to connect to their centre, so it is a wonderful thing to know the ISDN setup, troubleshooting and passing over the CCNA for your real world career. Let’s take a look at and how to prevent five common ISDN errors.

Recall that the telephone number on your dialer map is the distant router’s telephone number rather than the local one. Look at this – you don’t pick up your cell to dial your own number if you want to call a friend on your cell!
When you talk about the dialer map statements, at the end of the command, don’t forget the most significant diffusion option:

R1(config-if)#dialer map R2 broadcast 555555 R1(config-if)#dialer map

Without the “broadcast” option, the router will accept that command, but path protocol updates and violas cannot pass across the line. (In frame relay map statements this command is also required to facilitate transmission of broadcasting and multi-tasking).

PAP is an authentication system for PPP with clear texts, and clear texts are a bad idea indeed. However, do not forget that PAP requires extra setup if you have to configure it – the ppp pap command for the username to be sent.

R1 R1 R1 (config-if)

The R1 CISCO password is #ppp pap sent-username
Before using PPP subcommands you must set encapsulation to PPP.

R1(s) R1(s)

Another useful reminder is the error message we received when configuring the transmitter command – a BRI line, by default, runs HDLC rather than PPP. As HDLC does not permit us either to use PAP or CHAP, the ppp encapsulation command must set a PPP link.

R1(config-if)#encapsulation ppp

R1(config-if)#ppp authentication pap

R1(config-if)#ppp pap sent-username R1 password CISCO

But before we configure any of this information, we should configure the ISDN switch-type. Why? Because without the switch-type configuration, it doesn’t matter that we avoid the other four errors – the line will not come up. Configure the switch-type with the “isdn switch-type” command, and then verify it with “show isdn status”.

Ppp encapsulation R1(config-if)#

Authentication pap R1(config-if)#ppp

R1(config-if)#ppp pap CISCO password sent-username R1

We should, however, configure the ISDN switch type before configuring any of this information. What is the reason? It is not important that we prevent the other four errors without the setup of the switch-type – it does not occur in the line. Set a switch type to the command “isdn switch type” and then check it to “show isdn status.”
The basic-ni switch-type R1(config)#isdn

The status of R1#

Type of ISDN Global = base-ni (Output for clarity cut off this command here)

If you don’t remember this part of your setup, the show output is no time to remember!

The status of R1#

**** No currently specified **** Global ISDN Switch type

In your CCNA studies ISDN is an essential part, and this information is also useful in production networks. Continue to research, note the particulars, run those debugs, and before you know it you will be a CCNA!

Leave a Comment