Actual Cisco routers allow you to practice and learn display and debug commands, which is why I recommend getting your hands on real Cisco equipment rather than a simulator.
The knowledge you gain from debugging is priceless. To be honest, it’s this expertise that sets you apart from the “typical” CCNA who lacks hands-on experience. Observing debugs in action might also help you prepare for the CCNP. Because 90 to 95 percent of CCNAs go on to pursue the CCNP, getting started with debugs now is a smart idea.
Don’t put off learning debugs and shows until you’re studying for your CCNP or CCIE. Your work for the CCNA lays the groundwork for all else you’ll undertake in the future.
Never, ever, ever, ever, EVER, EVER, EVER, EVER, EVER, EVER, EVER, EVER, EVER, EVER There are some debugs that provide so much information that the router becomes overwhelmed and freezes up. Debugging should never be done on a live network.
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For a variety of reasons, knowing the right show and debug commands for ISDN is essential. First, you may observe how ISDN works and gain a better understanding of what’s going on by viewing it in action. Second, troubleshooting ISDN without understanding the necessary show and debug instructions is difficult, if not impossible. (It’s possible to miss an ISDN authentication fault simply by glancing at the setup, but debug ppp negotiation will rapidly reveal the source of the problem.)
Let’s look at the ISDN show and debug commands that every CCNA and CCNP should be familiar with.
Display ISDN Status
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It has to be this ISDN show command if you only know one. After configuring your ISDN switch type and any necessary SPIDs, always use this command. The command will display the switch type (and whether or not you configured a switch type), as well as whether or not the SPIDs you entered are valid:
- R1#show ISDN status is a command that displays the current status of an ISDN.
- basic-ni is the global ISDN switch type.
- BRI0 ISDN interface
- Switchtype = basic-ni dsl 0, interface ISDN
- Status of Layer 1:
ACTIVE
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Status of Layer 2:
- State = MULTIPLE FRAME ESTABLISHED, TEI = 91, Ces = 1, SAPI = 0.
- State = MULTIPLE FRAME ESTABLISHED, TEI = 92, Ces = 2, SAPI = 0.
Status of the Spid:
- TEI 91, state = 5, ces = 1 (init)
- spid1 is configured, there is no LDN, spid1 is sent, and spid1 is legitimate.
- epsf = 0, usid = 1, tid = 1 are the endpoint IDs.
- TEI 92, state = 5, ces = 2 (init)
- spid2 is configured, there is no LDN, spid2 is sent, and spid2 is legitimate.
- epsf = 0, usid = 3, tid = 1 are the endpoint IDs.
- Status of Layer 3:
- 0 Layer 3 Calls Active (s)
- CCBs = 0 Activated dsl
- 0x80000003 is the Free Channel Mask.
- You’ll get this output from show isdn status every now and then:
- R2#isdnstatus shows the current status of the ISDN.
- ISDN Switchtype = basic-ni1 is the current ISDN Switch type.
- BRI0 ISDN interface
Status of Layer 1:
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ACTIVE
Status of Layer 2:
NOT ACTIVATED LAYER 2
Status of the Spid:
- TEI Not Assigned, state = 3, ces = 1 (await establishment)
- spid1 is configured, but there is no LDN, spid1 is not sent, and spid1 is not valid.
- TEI Not Assigned, state = 1, ces = 2 (terminal down)
- spid2 is configured, but there is no LDN, spid2 is not communicated, and spid2 is not valid.
Check your current configuration, and if the SPIDs appear to be correct, simply close and reopen the BRI interface. After that, the run show ISDN status once more. If you see’spids are validâ after that, you’re good to go. If the notification “spids are invalid” still appears, you’ve most certainly mistyped the SPID.
Display the Access-List
What exactly does this command have to do with ISDN? Everything.
Remember how the ISDN connection was established in the first place? It’s an interesting flow of traffic. There is no interesting traffic by default. The dialer-list and dialer-group commands, as well as the access-list command, are used to specify interesting traffic. If your link isn’t showing up or isn’t staying up, use this command to check what traffic has been marked as interesting.
Display Dialer
Another command that can be used to figure out why an ISDN link is coming up and staying up. This command displays the number of successful and failed calls, as well as the current idle-timer value (120 seconds by default) and, most importantly, the source and destination of the current interesting traffic:
R1#dialer#dialer#dialer#dialer#dialer#dial
BRI0 – ISDN dialer type
Successes with the Dial String Failures Last contacted Status
8358662 1 0 00:00:59 8358662 1 0 00:00:59
successful
There have been no incoming call(s) that have been screened.
For the callback, 0 incoming call(s) were denied.
BRI0:1 – ISDN dialer type
120-second idle timer; 120-second idle timer; 120-second idle timer; 120-second idle timer; 120-second idle timer; (20 secs)
Re-enable after waiting 30 seconds for the carrier (15 secs)
The state of the dialer is data link layer up.
IP (s=172.12.21.1, d=172.12.21.2) is the dialing reason.
Time till the connection is lost 62 second
8358662 is a phone number that has been connected to you (R2)
The idle-timer value is set to its default, there are 62 seconds till the connection goes down (unless interesting traffic resets the timer), and the source and destination of the interesting traffic are 172.12.21.1 and 172.12.21.2, respectively. If the destination is a routing protocol’s multicast address, such as 224.0.0.5 for OSPF, you know what traffic is keeping the lineup.
History of ISDN
Do you want to know how many calls were made in the last 15 minutes? Simply execute this command. When you enter into an ISDN troubleshooting situation and want to know what calls have been made, this is useful.
R1#isdn#isdn#isdn#isdn#isd
history
———————————————————————————————————————— HISTORY OF ISDN CALLS ———————————————————————————————————————— The maximum number of entries in a history table is 100. Data from the history table is kept for a maximum of 15 minutes. ———————————————————————————————————————— Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minutes Minute Number of the kind. Idle Units/Currency Units/Currency Units/Currency Units/Currency Units/C —————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————
PPP Negotiation should be debugged.
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You must master this command not just for your CCNA and CCNP exams, but also to be an efficient ISDN troubleshooter.
It’s easy to mistype a password or neglect to configure a ‘username / passwordâ combination while configuring PPP authentication for the first time. Debug ppp negotiation and send a ping to bring the line up instead of constantly reviewing your running setup to figure out what’s wrong. You’ll immediately figure out what’s wrong.
R2#ppp-negotiation-debug
Debugging of the PPP protocol is now underway.
172.12.21.1 R2#ping
BR0:1 PPP: Phase is AUTHENTIFYING for both parties.
BR0:1 CHAP: O CHALLENGE id 1 len 23 from “R2” 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: I CHALLENGE id 1 len 23 from “R1”
BR0:1 CHAP: O RESPONSE id 1 len 23 from “R2” BR0:1 CHAP: O RESPONSE id 1 len 23 from “R2”
BR0:1 CHAP: I SUCCESS id 1 len 4 BR0:1 CHAP: I SUCCESS
BR0:1 CHAP: I RESPONSE id 1 len 23 from “R1” BR0:1 CHAP: I RESPONSE id 1 len 23 from “R1”
BR0:1 CHAP: O SUCCESS id 1 len 4 BR0:1 CHAP: O SUCCESS id 1 len 4 BR0:1 CHAP
By grasping these basic ISD concepts, you’ll be well on your way to become a
Chris Bryant, CCIE #12933, is the founder of The Bryant Advantage, which offers over 100 free certification exam courses, including Cisco CCNA test prep articles. Also available is his Cisco CCNA study guide and Cisco CCNA training!
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