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| 4.4 |  | Troubleshooting ISDN |
| 4.4.3 |  | Troubleshooting ISDN BRI to telco communications |

ISDN problems generally fall into one of the following categories:

- Communications problems between the router and the telco’s ISDN switch
- Logical configuration problems with dialers
- Problems with the number being called

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.