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Cisco CCNA Exam Tutorial: Five ISDN Details To Remember |
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CCNA exam success depends on mastering many technologies
that are new to you, and few exam topics have more details
than ISDN. ISDN isn't just for your CCNA exam studies, though.
While ISDN is dismissed by many, the fact is that there are
many small and mid-size networks out there that use ISDN as
their backup to frame relay. Some of these companies have
spoke networks that use ISDN to connect to their hub as well,
so it's a great idea to know ISDN configuration and troubleshooting
for your real-world career as well as passing the CCNA. With
that in mind, let's take a look at five common ISDN errors
and how to avoid them.
With dialer map statements, remember that the phone number
you put in the dialer map is the phone number of the remote
router, not the local one. Look at it this way - if you want
to call a friend on your cell, you don't pick up your cell
and dial your own number!
Speaking of dialer map statements, don't forget the all-important
broadcast option at the end of the command:
R1(config-if)#dialer map ip 172.12.21.1 name R2 broadcast
5555555
The router will accept that command without the "broadcast"
option, but routing protocol updates and hellos would not
be able to travel across the line. (This command is also needed
in frame relay map statements to allow broadcasts and multicasts
to be transmitted.)
PAP is PPP's clear-text authentication scheme, and clear
text is a really bad idea. But if you do have to configure
it, don't forget that PAP requires additional configuration
-the ppp pap sent-username command.
R1(config-if)#ppp pap sent-username R1 password CISCO
Must set encapsulation to PPP before using PPP subcommands
R1(config-if)#
The error message we got while configuring the sent-username
command is another important reminder - by default, a BRI
line is running HDLC, not PPP. Since HDLC doesn't allow us
to use either PAP or CHAP, we'll need to set the link to PPP
with the encapsulation ppp command.
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".
R1(config)#isdn switch-type basic-ni
R1#show isdn status
Global ISDN Switchtype = basic-ni (output of this command
cut here for clarity)
If you forget this part of the configuration, the output
of show isdn status wastes no time in reminding you!
R1#show isdn status
**** No Global ISDN Switchtype currently defined ****
ISDN is an important part of your CCNA studies, and this
knowledge still comes in handy in production networks as well.
Keep studying, notice the details, run those debugs, and you'll
be a CCNA before you know it!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages.
For a FREE copy of his latest e-books, “How To Pass
The CCNA” and “How To Pass The CCNP”, visit
the website and download your free copies. You can also get
FREE CCNA and CCNP exam questions every day! Pass the CCNA
exam with The Bryant Advantage!
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