Cisco Certification: Debug and Show commands for ISDN

Written By: Chris Bryant

The major reason I recommend getting your hands on real Cisco equipment rather than a simulator is that real Cisco routers give you the chance to practice and learn show and debug commands.

The knowledge you acquire from debugs is invaluable. Frankly, its this knowledge that puts you above the average CCNA who doesnt have that hands-on experience. Watching debugs in action also gives you a head start on the CCNP. Since 90 95% of CCNAs go on to pursue the CCNP, its a great idea to get started with debugs now.

Dont make the mistake of waiting until youre studying for your CCNP and CCIE to start learning debugs and shows. The work you do for the CCNA is the foundation for everything youll do in the future.

Never, ever, ever practice debugs on a production network. There are debugs that will give you so much information that the router actually becomes overloaded and then locks up. Never practice debugs on a production network.

Its important to know the proper show and debug commands for ISDN for several reasons. First, by watching ISDN in operation, you can see its processes and better understand whats going on. Secondly, its difficult if not impossible to properly troubleshoot ISDN without knowing the proper show and debug commands. (Its easy to overlook an ISDN authentication error just by looking at the configuration, but running debug ppp negotiation will quickly show you where the problem lies.)

Lets take a look at the ISDN show and debug commands that every CCNA and CCNP should know.

Show ISDN Status

If you only know one ISDN show command, its got to be this one. Always use this command after configuring your ISDN switch type and any necessary SPIDs. The command will show you the switch type (and will also show you if you did not configure a switch type), and whether the SPIDs you entered are valid:

 R1#show isdn status Global ISDN Switchtype = basic-ni     ISDN BRI0 interface         dsl 0, interface ISDN Switchtype = basic-ni     Layer 1 Status:         ACTIVE     Layer 2 Status:         TEI = 91, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED         TEI = 92, Ces = 2, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED     Spid Status:         TEI 91, ces = 1, state = 5(init)             spid1 configured, no LDN, spid1 sent, spid1 valid                Endpoint ID Info: epsf = 0, usid = 1, tid = 1         TEI 92, ces = 2, state = 5(init)             spid2 configured, no LDN, spid2 sent, spid2 valid                Endpoint ID Info: epsf = 0, usid = 3, tid = 1     Layer 3 Status:         0 Active Layer 3 Call(s)     Activated dsl 0 CCBs = 0     The Free Channel Mask:  0x80000003 

Once in a while, youll get this output from show isdn status:

 R2#show isdn status The current ISDN Switchtype = basic-ni1 ISDN BRI0 interface     Layer 1 Status:         ACTIVE     Layer 2 Status:         Layer 2 NOT Activated     Spid Status:         TEI Not Assigned, ces = 1, state = 3(await establishment)             spid1 configured, no LDN, spid1 NOT sent, spid1 NOT valid         TEI Not Assigned, ces = 2, state = 1(terminal down)             spid2 configured, no LDN, spid2 NOT sent, spid2 NOT valid 

Check your running configuration, and if the SPIDs look good, simply close the BRI interface and open it again. Then run show ISDN status again. If you then see spids are valid, youre ready to proceed. If you still see a message that the spids are invalid, youve most likely mistyped the SPID.

Show Access-List

Whats this command got to do with ISDN? Everything.

Remember how the ISDN link comes up in the first place? Interesting traffic. By default, there is no interesting traffic. You define interesting traffic with the dialer-list and dialer-group commands, AND the access-list command. If you have a problem with your link never coming up or with it coming up and staying up, use this command to see what traffic has been defined as interesting.

Show Dialer

Another helpful command to determine why an ISDN link is coming up and staying up. This command shows you how many successful calls and failed calls have taken place, what the current idle-timer value is (by default, its 120 seconds), and most importantly, what the source and destination was for the current interesting traffic:

 R1#show dialer BRI0 - dialer type = ISDN Dial String      Successes   Failures    Last called   Last status 8358662                  1                 0           00:00:59       successful 0 incoming call(s) have been screened. 0 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: ip (s=, d=    Time until disconnect 62 secs     Connected to 8358662 (R2) 

Here, you can see that the idle-timer value is at its default, that there are 62 seconds left until the link comes down (unless interesting traffic resets the timer), and that the source of the interesting traffic was and the destination is If that destination is the multicast address of a routing protocol say, for OSPF you know what traffic is keeping the line up.

Show ISDN History

Want to see what calls have been made in the last 15 minutes? Just run this command. Its helpful if youre walking in to an ISDN troubleshooting situation and want to see what calls have been made.

R1#show isdn history——————————————————————————– ISDN CALL HISTORY——————————————————————————–History table has a maximum of 100 entries.History table data is retained for a maximum of 15 Minutes.——————————————————————————–Call Calling Called Remote Seconds Seconds Seconds ChargesType Number .Number Name Used Left Idle Units/Currency———————————————————————————————————–Out 8358662 R2 121 0Out 8358662 R2 121 0—————————————————————————————————–

Debug PPP Negotiation

Not only do you need to know this command for your CCNA and CCNP exams, you MUST know it to be an effective ISDN troubleshooter.

When PPP authentication is first configured, its simple to mistype a password, or forget to configure a username / password combination. Instead of continually reading your running configuration to see what the problem is, run debug ppp negotiation and send a ping to bring the line up. Youll quickly see where the problem is.

 R2#debug ppp negotiation PPP protocol negotiation debugging is on R2#ping  BR0:1 PPP: Phase is AUTHENTICATING, by both  BR0:1 CHAP: O CHALLENGE id 1 len 23 from "R2"  BR0:1 CHAP: I CHALLENGE id 1 len 23 from "R1"  BR0:1 CHAP: O RESPONSE id 1 len 23 from "R2"  BR0:1 CHAP: I SUCCESS id 1 len 4  BR0:1 CHAP: I RESPONSE id 1 len 23 from "R1"  BR0:1 CHAP: O SUCCESS id 1 len 4 

By mastering these simple ISDN show and debug commands, you increase your chances of passing the CCNA and CCNP exams greatly, and vastly improve your on-the-job skills.

I recommend you visit www.cisco.com/univercd for more show and debug commands. Learning to negotiate this valuable online resource will truly help you become more valuable on the job. Every Cisco router and switch command, along with default and usage guidelines, can be found on this site. Ill have an article posted soon to help you learn your way around the site; in the meantime, I urge you to begin adding this website to your troubleshooting toolkit.

Chris Bryant
CCIE #12933

About The Author

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage. The Bryant Advantage’s website offers FREE ebooks and tutorials for the CCNA and CCNP exams, FREE subscriptions to “Cisco Certification Central”, and sells the best CCNA and CCNP prep courses and books on the market today. Visit his site at www.thebryantadvantage.com today!


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