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, it’s this knowledge that puts you above the “average" CCNA who doesn’t 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, it’s a great idea to get started with debugs now.

Don’t make the mistake of waiting until you’re studying for your CCNP and CCIE to start learning debugs and shows. The work you do for the CCNA is the foundation for everything you’ll 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.

It’s 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 what’s going on. Secondly, it’s difficult if not impossible to properly troubleshoot ISDN without knowing the proper show and debug commands. (It’s 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.)

Let’s 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, it’s 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 switch type is correctly configured.>

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 < SPID 1 is good.>

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, you’ll 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", you’re ready to proceed. If you still see a message that the spids are invalid, you’ve most likely mistyped the SPID.

Show Access-List

What’s 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, it’s 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=172.12.21.1, d=172.12.21.2) < s=Source, d=Destination >

Time until disconnect 62 secs

< Idle timer, reset by interesting traffic only >

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 172.12.21.1 and the destination is 172.12.21.2. If that destination is the multicast address of a routing protocol – say, 224.0.0.5 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. It’s helpful if you’re 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 Charges Type Number .Number Name Used Left Idle Units/Currency ----------------------------------------------------------------------------------------------------------- Out 8358662 R2 121 0 Out 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, it’s 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. You’ll quickly see where the problem is.

R2#debug ppp negotiation

PPP protocol negotiation debugging is on

R2#ping 172.12.21.1

BR0:1 PPP: Phase is AUTHENTICATING, by both

< Both routers are authenticating the other with CHAP. >

BR0:1 CHAP: O CHALLENGE id 1 len 23 from "R2"

< R2 is sending a challenge to the remote router. O = “outgoing". >

BR0:1 CHAP: I CHALLENGE id 1 len 23 from "R1"

< A CHAP challenge has come in from R1. I = “incoming">

BR0:1 CHAP: O RESPONSE id 1 len 23 from "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 challenge was successfully met. >

BR0:1 CHAP: I RESPONSE id 1 len 23 from "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 success message is sent. >

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. I’ll 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

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Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of over 100 free certification exam tutorials, including 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!