A divide and conquer approach is the best way to troubleshoot ISDN.
The show isdn status command will quickly determine whether
the router is communicating correctly with telco’s ISDN switch. If there is a
problem in this regard, the show isdn status command will
often provide helpful diagnostic information.
In particular Layer 1
should be active, Layer 2 should have MULTIPLE_FRAME_ESTABLISHED and SPIDs
should be valid.

If Layer 1
is deactivated then troubleshoot the following physical layer problems:

- Physical cabling
- Interface is activated (no shutdown)
- ISDN Switch type
Like primary rate interfaces, basic rate interfaces must be told what
type of ISDN switching equipment is used by the telco. The configured ISDN
switch type can also be determined with the show isdn
status command.

In Figure
, no switch type
has been configured. A misconfigured switch type can result in either Layer 1
or Layer 1/2 status "NOT Activated". In early versions of the Cisco
IOS the switch type was configured from global configuration, and a router
could only be used with one ISDN switch type service. Later versions of the IOS
allow the switch type to be specified at an interface level, removing this
constraint.
If Layer 1 is functioning but Layer 2 is inactive, then
check the ISDN switch type.

If SPID1
or SPID2 are not valid, carefully check the configured SPIDs and verify these
with the telco.
It may be
necessary to shutdown the BRI and then reactivate it or reboot the router to
force SPID negotiation after the configuration is changed. There is also a
known bug in some versions of the IOS that reports an invalid SPID even though
the configured value is correct. As long as the BRI is able to make calls, this
"cosmetic" bug should not be reason for concern.
In certain parts of the world, notably in Europe, telco ISDN switches may
deactivate Layer 1 or 2 when there are no active calls. Hence, when there are
no active calls, show isdn status will indicate the Layer 1 and 2 are down. But
when a call occurs, Layers 1 and 2 will be brought up. Make a test BRI call to
verify whether the BRI is functioning. If the call succeeds, then no further
ISDN troubleshooting is required.
Layer 3
Status:
TWAIT timer active
0 Active Layer 3
Call(s)
Activated dsl 0 CCBs = 0
The Free
Channel Mask: 0x80000003
Total Allocated ISDN CCBs =
0
Reboot the ISDN router by cycling the power. When the
router comes up, the BRI interface will be unable to establish a call for a
random period of time. Executing the show isdn status
command will reveal that the TWAIT timer is active. This is a feature that
prevents a telco’s ISDN switch from being overloaded after a major power outage
is restored and multiple ISDN devices simultaneously try to establish a
connection.
The timer can be cleared by manually shutting down the BRI
interface and reactivating it.
The delayed behavior can be changed using
the isdn twait-disable interface configuration command.
The debug isdn q921 command can also be used to
troubleshoot Layer 2 isdn issues.
This command
will show the Layer 2 exchanges between the router and the telco’s ISDN switch.
However, the show isdn status command will normally be of
more use as it effectively decodes the Q921 messages and displays them in a
more understandable format.