Cisco CCNA / CCNP CIT Training: Troubleshooting ISDN

By Chris Bryant, CCIE #12933

When you start configuring ISDN, you're concerned with getting the interesting traffic defined, maybe configuring PAP or CHAP authentication, or any of the other PPP features -- and it's really easy to forget the basics.

The first basic is defining the ISDN switchtype. Forgetting this is a real pain, because the router will allow you to write the entire ISDN configuration without telling you that you didn't configure the switchtype! The command that will tell you is show isdn status.

R1#show isdn status
**** No Global ISDN Switchtype currently defined ****
ISDN BRI0/0 interface
dsl 0, interface ISDN Switchtype = none
Layer 1 Status:
DEACTIVATED
Layer 2 Status:
Layer 2 NOT Activated
Layer 3 Status:
0 Active Layer 3 Call(s)

When a Cisco router puts four asterisks at both ends of a message, you know it's important! Actually, this goes beyond important, because your ISDN config won't work until the proper switchtype is configured. Your service provider will tell you the proper type, and IOS Help shows you all your options as well. You can configure the switchtype globally, as shown here, or on the interface itself.

R1(config)#isdn switch-type ?
basic-1tr6 1TR6 switch type for Germany
basic-5ess Lucent 5ESS switch type for the U.S.
basic-dms100 Northern Telecom DMS-100 switch type for the U.S.
basic-net3 NET3 switch type for UK, Europe, Asia and Australia
basic-ni National ISDN switch type for the U.S.
basic-qsig QSIG switch type
basic-ts013 TS013 switch type for Australia (obsolete)
ntt NTT switch type for Japan
vn3 VN3 and VN4 switch types for France

R1#show isdn status
Global ISDN Switchtype = basic-ni

The other commonly forgotten value is the Service Provider ID, or SPID. Not all ISDN configurations require these - again, the service provider will tell you whether these need to be configured. Below, I'm entering two SPIDs on a BRI interface that's been previously opened. Always verify SPIDs with show isdn status.

R1(config)#int bri0/0
R1(config-if)#isdn spid1 0555100001
R1(config-if)#isdn spid2 0555300001

R1#show isdn status
Global ISDN Switchtype = basic-ni
ISDN BRI0/0 interface
dsl 0, interface ISDN Switchtype = basic-ni
Layer 1 Status:
ACTIVE
Layer 2 Status:
TEI = 124, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED
TEI 124, ces = 1, state = 8(established)
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

There's some good news here in that Layer 2 status shows as MULTIPLE_FRAME_ESTABLISHED, which is what we want to see. What we don't want to see is spid1 NOT valid and spid2 NOT valid. Before you start driving yourself nuts squinting at the screen to wonder which number you mistyped, try closing and reopening the interface.

R1(config)#int bri0/0
R1(config-if)#shut
*Mar 1 00:29:38.595: %ISDN-6-LAYER2DOWN: Layer 2 for Interface BR0/0, TEI 124 c
hanged to down
R1(config-if)#no shut

*Mar 1 00:29:38.647: %LINK-5-CHANGED: Interface BRI0/0, changed state to admini
stratively down
*Mar 1 00:29:39.671: %LINK-3-UPDOWN: Interface BRI0/0:1, changed state to down
*Mar 1 00:29:39.671: %LINK-3-UPDOWN: Interface BRI0/0:2, changed state to down
*Mar 1 00:29:39.715: %ISDN-6-LAYER2UP: Layer 2 for Interface BR0/0, TEI 124 cha
nged to up
*Mar 1 00:29:39.723: %LINK-3-UPDOWN: Interface BRI0/0, changed state to up
R1(config-if)#
*Mar 1 00:29:39.947: %ISDN-6-LAYER2UP: Layer 2 for Interface BR0/0, TEI 65 chan
ged to up

R1#show isdn status
Global ISDN Switchtype = basic-ni
ISDN BRI0/0 interface
dsl 0, interface ISDN Switchtype = basic-ni
Layer 1 Status:
ACTIVE
Layer 2 Status:
TEI = 124, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED
TEI = 65, Ces = 2, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED
TEI 124, ces = 1, state = 5(init)
spid1 configured, no LDN, spid1 sent, spid1 valid
Endpoint ID Info: epsf = 0, usid = 1, tid = 1
TEI 65, ces = 2, state = 5(init)
spid2 configured, no LDN, spid2 sent, spid2 valid
Endpoint ID Info: epsf = 0, usid = 3, tid = 1

Rule #1 of ISDN troubleshooting: ALWAYS check the ISDN switchtype and SPIDs first! (You could also run debug isdn q921 to check L2 ISDN activity, but when I see SPIDs showing as inactive, the first thing I do is reset the BRI interface.)

You may notice that the Terminal End Indicator (TEI) will change on some ISDN configurations when the interface is shut and reopened, and will remain static on others. That's because TEI values can be static or dynamic. Here are the ranges you should know:

Static: 0 - 63 (You'll only see zero in PRI deployments.)

Dynamic: 64 - 126

Management TEI: 127 (Broadcast TEI)

Basic ISDN troubleshooting is an important part of both your CCNA and CCNP studies. Don't let anyone tell you that ISDN is "ancient" - there's a lot of it still out there in today's networks, and you better know how to configure and troubleshoot it!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com