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The major reason I recommend g tting your hands on real Cisco quipment rather than a simulator is th t real Cisco routers give you the ch nce to practice and learn show and d bug commands. The knowledge you acquire fr m debugs is invaluable. Frankly, it's th s knowledge that puts you above the ' verage” CCNA who doesn't have that h nds-on experience. Watching debugs in action lso gives you a head start on the CCNP. S nce 90 - 95% of CCNAs go on to p rsue the CCNP, it's a great dea to get started with debugs n w. Don't make the mistake of w iting until you're studying for your CCNP and CCIE to st rt learning debugs and shows. The w rk you do for the CCNA is the f undation for everything you'll do in the f ture. Never, ever, ever practice debugs on a pr duction network. There are debugs that w ll give you so much information th t the router actually becomes overloaded and th n locks up. Never practice debugs on a pr duction network. It's important to know the pr per show and debug commands for ISDN for s veral reasons. First, by watching ISDN in peration, you can see its processes and b tter understand what's going on. Secondly, t's difficult if not impossible to pr perly troubleshoot ISDN without knowing the pr per show and debug commands. (It's asy to overlook an ISDN authentication rror just by looking at the c nfiguration, but running debug ppp negotiation w ll quickly show you where the pr blem lies.)
Let's take a look at the ISDN sh w and debug commands that every CCNA and CCNP sh uld know. Show ISDN Status If you nly know one ISDN show command, t's got to be this one. Alw ys use this command after configuring y ur ISDN switch type and any n cessary SPIDs. The command will show you the sw tch type (and will also show you if you did not c nfigure a switch type), and whether the SPIDs you ntered are valid: R1#show isdn status Gl bal ISDN Switchtype = basic-ni < ISDN switch type is correctly configured.>
ISDN BRI0 nterface dsl 0, interface ISDN Switchtype = b sic-ni Layer 1 Status: ACTIVE Layer 2 St tus: TEI = 91, Ces = 1, SAPI = 0, St te = MULTIPLE_FRAME_ESTABLISHED TEI = 92, Ces = 2, SAPI = 0, St te = MULTIPLE_FRAME_ESTABLISHED Spid Status: TEI 91, ces = 1, st te = 5(init) spid1 configured, no LDN, sp d1 sent, spid1 valid < SPID 1 is good.> Endpoint ID Inf : epsf = 0, usid = 1, tid = 1 TEI 92, ces = 2, st te = 5(init) spid2 configured, no LDN, sp d2 sent, spid2 valid Endpoint ID Inf : epsf = 0, usid = 3, tid = 1 L yer 3 Status: 0 Active Layer 3 C ll(s) Activated dsl 0 CCBs = 0 The Fr e Channel Mask: 0x80000003 Once in a wh le, you'll get this output from sh w isdn status: R2#show isdn status
The c rrent ISDN Switchtype = basic-ni1
ISDN BRI0 nterface Layer 1 Status: ACTIVE Layer 2 St tus: Layer 2 NOT Activated Spid St tus: TEI Not Assigned, ces = 1, st te = 3(await establishment) spid1 configured, no LDN, sp d1 NOT sent, spid1 NOT valid TEI Not Ass gned, ces = 2, state = 1(t rminal down) spid2 configured, no LDN, sp d2 NOT sent, spid2 NOT valid Ch ck your running configuration, and if the SPIDs l ok good, simply close the BRI nterface and open it again. Then run sh w ISDN status again. If you th n see 'spids are valid”, you're r ady to proceed. If you still see a m ssage that the spids are invalid, y u've most likely mistyped the SPID.
Show Access-List What's this command got to do w th ISDN? Everything. Remember how the ISDN l nk comes up in the first pl ce? Interesting traffic. By default, there is no nteresting traffic. You define interesting traffic w th the dialer-list and dialer-group commands, AND the ccess-list command. If you have a pr blem with your link never coming up or w th it coming up and staying up, use th s command to see what traffic has b en defined as interesting. Show Dialer An ther helpful command to determine why an ISDN l nk is coming up and staying up. Th s command shows you how many s ccessful calls and failed calls have t ken place, what the current idle-timer v lue is (by default, it's 120 s conds), and most importantly, what the s urce and destination was for the c rrent interesting traffic: R1#show dialer
BRI0 - d aler type = ISDN
D al String Successes F ilures Last called Last st tus
8358662 1 0 00:00:59 s ccessful
0 incoming call(s) h ve been screened.
0 ncoming call(s) rejected for callback. BRI0:1 - d aler type = ISDN
Idl timer (120 secs), Fast idle t mer (20 secs)
Wait for c rrier (30 secs), Re-enable (15 secs)
D aler state is data link layer up
D al reason: ip (s=172.12.21.1, d=172.12.21.2) < s=Source, d=Destination >
T me until disconnect 62 secs
< Idle timer, reset by interesting traffic only >
C nnected to 8358662 (R2) Here, you can see th t the idle-timer value is at its d fault, that there are 62 seconds l ft until the link comes down ( nless interesting traffic resets the timer), and th t the source of the interesting tr ffic was 172.12.21.1 and the destination is 172.12.21.2. If th t destination is the multicast address of a r uting protocol - say, 224.0.0.5 for OSPF - you kn w what traffic is keeping the l ne up. Show ISDN History Want to see wh t calls have been made in the l st 15 minutes? Just run this c mmand. It's helpful if you're walking in to an ISDN tr ubleshooting situation and want to see wh t calls have been made. R1#show sdn history-------------------------------------------------------------------------------- ISDN CALL HISTORY--------------------------------------------------------------------------------H story table has a maximum of 100 ntries.History table data is retained for a m ximum of 15 Minutes.--------------------------------------------------------------------------------Call Calling C lled Remote Seconds S conds Seconds ChargesType Number .N mber Name Used L ft Idle Un ts/Currency-----------------------------------------------------------------------------------------------------------Out 8358662 R2 121 0O t 8358662 R2 121 0----------------------------------------------------------------------------------------------------- D bug PPP Negotiation Not only do you n ed to know this command for y ur CCNA and CCNP exams, you MUST kn w it to be an effective ISDN tr ubleshooter. When PPP authentication is first c nfigured, it's simple to mistype a p ssword, or forget to configure a ' sername / password” combination. Instead of c ntinually reading your running configuration to see wh t the problem is, run debug ppp n gotiation and send a ping to br ng the line up. You'll quickly see wh re the problem is. R2#debug ppp n gotiation
PPP protocol negotiation debugging is on
R2#p ng 172.12.21.1 BR0:1 PPP: Phase is AUTHENTICATING, by b th
< Both routers are authenticating the other with CHAP. >
BR0:1 CHAP: O CHALLENGE id 1 len 23 fr m "R2"
< R2 is sending a challenge to the remote router. O = 'outgoing”. >
BR0:1 CHAP: I CHALLENGE id 1 len 23 fr m "R1"
< A CHAP challenge has come in from R1. I = 'incoming”>
BR0:1 CHAP: O RESPONSE id 1 len 23 fr m "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 ch llenge was successfully met. >
BR0:1 CHAP: I RESPONSE id 1 len 23 fr m "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 s ccess message is sent. > By m stering these simple ISDN show and d bug commands, you increase your chances of p ssing the CCNA and CCNP exams gr atly, and vastly improve your on-the-job sk lls. I recommend you visit www.cisco.com/univercd for m re show and debug commands. Learning to n gotiate this valuable online resource will tr ly help you become more valuable on the j b. Every Cisco router and switch c mmand, along with default and usage g idelines, can be found on this s te. I'll have an article posted s on to help you learn your way round the site; in the meantime, I rge you to begin adding this w bsite 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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