The major reason I recommend gtting your hands on real Cisco quipment rather than a simulator is tht real Cisco routers give you the chnce to practice and learn show and dbug commands.

The knowledge you acquire frm debugs is invaluable. Frankly, it's ths knowledge that puts you above the 'verage” CCNA who doesn't have that hnds-on experience. Watching debugs in action lso gives you a head start on the CCNP. Snce 90 - 95% of CCNAs go on to prsue the CCNP, it's a great dea to get started with debugs nw.

Don't make the mistake of witing until you're studying for your CCNP and CCIE to strt learning debugs and shows. The wrk you do for the CCNA is the fundation for everything you'll do in the fture.

Never, ever, ever practice debugs on a prduction network. There are debugs that wll give you so much information tht the router actually becomes overloaded and thn locks up. Never practice debugs on a prduction network.

It's important to know the prper show and debug commands for ISDN for sveral reasons. First, by watching ISDN in peration, you can see its processes and btter understand what's going on. Secondly, t's difficult if not impossible to prperly troubleshoot ISDN without knowing the prper show and debug commands. (It's asy to overlook an ISDN authentication rror just by looking at the cnfiguration, but running debug ppp negotiation wll quickly show you where the prblem lies.)

Let's take a look at the ISDN shw and debug commands that every CCNA and CCNP shuld know.

Show ISDN Status

If you nly know one ISDN show command, t's got to be this one. Alwys use this command after configuring yur ISDN switch type and any ncessary SPIDs. The command will show you the swtch type (and will also show you if you did not cnfigure a switch type), and whether the SPIDs you ntered are valid:

R1#show isdn status

Glbal ISDN Switchtype = basic-ni < ISDN switch type is correctly configured.>
ISDN BRI0 nterface

dsl 0, interface ISDN Switchtype = bsic-ni

Layer 1 Status:

ACTIVE

Layer 2 Sttus:

TEI = 91, Ces = 1, SAPI = 0, Stte = MULTIPLE_FRAME_ESTABLISHED

TEI = 92, Ces = 2, SAPI = 0, Stte = MULTIPLE_FRAME_ESTABLISHED

Spid Status:

TEI 91, ces = 1, stte = 5(init)

spid1 configured, no LDN, spd1 sent, spid1 valid < SPID 1 is good.>

Endpoint ID Inf: epsf = 0, usid = 1, tid = 1

TEI 92, ces = 2, stte = 5(init)

spid2 configured, no LDN, spd2 sent, spid2 valid

Endpoint ID Inf: epsf = 0, usid = 3, tid = 1

Lyer 3 Status:

0 Active Layer 3 Cll(s)

Activated dsl 0 CCBs = 0

The Fre Channel Mask: 0x80000003

Once in a whle, you'll get this output from shw isdn status:

R2#show isdn status
The crrent ISDN Switchtype = basic-ni1
ISDN BRI0 nterface

Layer 1 Status:

ACTIVE

Layer 2 Sttus:

Layer 2 NOT Activated

Spid Sttus:

TEI Not Assigned, ces = 1, stte = 3(await establishment)

spid1 configured, no LDN, spd1 NOT sent, spid1 NOT valid

TEI Not Assgned, ces = 2, state = 1(trminal down)

spid2 configured, no LDN, spd2 NOT sent, spid2 NOT valid

Chck your running configuration, and if the SPIDs lok good, simply close the BRI nterface and open it again. Then run shw ISDN status again. If you thn see 'spids are valid”, you're rady to proceed. If you still see a mssage that the spids are invalid, yu've most likely mistyped the SPID.

Show Access-List

What's this command got to do wth ISDN? Everything.

Remember how the ISDN lnk comes up in the first plce? Interesting traffic. By default, there is no nteresting traffic. You define interesting traffic wth the dialer-list and dialer-group commands, AND the ccess-list command. If you have a prblem with your link never coming up or wth it coming up and staying up, use ths command to see what traffic has ben defined as interesting.

Show Dialer

Anther helpful command to determine why an ISDN lnk is coming up and staying up. Ths command shows you how many sccessful calls and failed calls have tken place, what the current idle-timer vlue is (by default, it's 120 sconds), and most importantly, what the surce and destination was for the crrent interesting traffic:

R1#show dialer

BRI0 - daler type = ISDN

Dal String Successes Filures Last called Last sttus

8358662 1 0 00:00:59
sccessful

0 incoming call(s) hve been screened.

0 ncoming call(s) rejected for callback.

BRI0:1 - daler type = ISDN

Idl timer (120 secs), Fast idle tmer (20 secs)

Wait for crrier (30 secs), Re-enable (15 secs)

Daler state is data link layer up

Dal reason: ip (s=172.12.21.1, d=172.12.21.2) < s=Source, d=Destination >

Tme until disconnect 62 secs

< Idle timer, reset by interesting traffic only >

Cnnected to 8358662 (R2)

Here, you can see tht the idle-timer value is at its dfault, that there are 62 seconds lft until the link comes down (nless interesting traffic resets the timer), and tht the source of the interesting trffic was 172.12.21.1 and the destination is 172.12.21.2. If tht destination is the multicast address of a ruting protocol - say, 224.0.0.5 for OSPF - you knw what traffic is keeping the lne up.

Show ISDN History

Want to see wht calls have been made in the lst 15 minutes? Just run this cmmand. It's helpful if you're walking in to an ISDN trubleshooting situation and want to see wht calls have been made.

R1#show sdn
history-------------------------------------------------------------------------------- ISDN CALL HISTORY--------------------------------------------------------------------------------Hstory table has a maximum of 100 ntries.History table data is retained for a mximum of 15 Minutes.--------------------------------------------------------------------------------Call Calling Clled Remote Seconds Sconds Seconds ChargesType Number .Nmber Name Used Lft Idle Unts/Currency-----------------------------------------------------------------------------------------------------------Out 8358662 R2 121 0Ot 8358662 R2 121 0-----------------------------------------------------------------------------------------------------

Dbug PPP Negotiation

Not only do you ned to know this command for yur CCNA and CCNP exams, you MUST knw it to be an effective ISDN trubleshooter.

When PPP authentication is first cnfigured, it's simple to mistype a pssword, or forget to configure a 'sername / password” combination. Instead of cntinually reading your running configuration to see wht the problem is, run debug ppp ngotiation and send a ping to brng the line up. You'll quickly see whre the problem is.

R2#debug ppp ngotiation
PPP protocol negotiation debugging is on
R2#png 172.12.21.1

BR0:1 PPP: Phase is AUTHENTICATING, by bth
< Both routers are authenticating the other with CHAP. >
BR0:1 CHAP: O CHALLENGE id 1 len 23 frm "R2"
< R2 is sending a challenge to the remote router. O = 'outgoing”. >
BR0:1 CHAP: I CHALLENGE id 1 len 23 frm "R1"
< A CHAP challenge has come in from R1. I = 'incoming”>
BR0:1 CHAP: O RESPONSE id 1 len 23 frm "R2"
< R2 is sending a response to the challenge. >
BR0:1 CHAP: I SUCCESS id 1 len 4
< A success message has come in , indicating the chllenge was successfully met. >
BR0:1 CHAP: I RESPONSE id 1 len 23 frm "R1"
< A response to the challenge sent to R1 is now coming in. >
BR0:1 CHAP: O SUCCESS id 1 len 4
< The challenge was successfully met by R1, and a sccess message is sent. >

By mstering these simple ISDN show and dbug commands, you increase your chances of pssing the CCNA and CCNP exams gratly, and vastly improve your on-the-job sklls.

I recommend you visit www.cisco.com/univercd for mre show and debug commands. Learning to ngotiate this valuable online resource will trly help you become more valuable on the jb. Every Cisco router and switch cmmand, along with default and usage gidelines, can be found on this ste. I'll have an article posted son to help you learn your way round the site; in the meantime, I rge you to begin adding this wbsite to your troubleshooting toolkit.

The article Cisco Certification: Introduction To ISDN, Part V was Submitted by Chris Bryant through Articles.GetACoder.com network. Here's the additional information: Chris Bryant, CCIE #12933, is the wner of The Bryant Advantage, home of ver 100 free certification exam tutorials, ncluding Cisco CCNA certification test prep articles. His exclusive Cisco CCNA study guide and Cisco CCNA training is also available! Visit his blog and sign up for Cisco Certification Central, a daily newsletter packed with CCNA, Network+, Security+, A+, and CCNP certification exam practice questions! A free 7-part course, "How To Pass The CCNA", is also available, and you can attend an in-person or online CCNA boot camp with The Bryant Advantage!

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