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Cisco Certification: Introduction To ISDN, Part V |
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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 'averageâ€
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 validâ€,
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 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, 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 = 'outgoingâ€.
> BR0:1 CHAP: I CHALLENGE id 1 len 23 from "R1"
< A CHAP challenge has come in from R1. I = 'incomingâ€>
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.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, visit the website and download
your free copies. You can also get FREE CCNA and CCNP exam
questions every day! Pass the CCNA exam with The Bryant Advantage!
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