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| 4.4 |  | Troubleshooting ISDN |
| 4.4.5 |  | Debugging ISDN call setup failures |

If it appears that the router is attempting to dial but calls are not getting through, the problem can be investigated further using the debug isdn q931 command. This command will provide information on the ISDN call status as reported to the router by the telco’s ISDN switch. A simple test is to execute debug isdn q931 on both routers to determine whether the far end router is actually receiving the call. Look for any q931 debug output. Further explicit information about the call failure will be provided in the debug messages. Although the debug output reports the basic reasons for a disconnect, more detailed information may be required.

> Aug 13 18:23:14.734: ISDN BR0: RX <- RELEASE_COMP pd = 8 callref = 0x86

> Aug 13 18:23:14.742: Cause i = 0x829C - Invalid number format (incomplete number)

The first most significant byte after 0x indicates where in the circuit path the Disconnected Cause Code was generated. In the example above, 82 indicates the call was disconnected from the local telco switch. The list below defines the Cause Code Origination Points to help interpret where the call was disconnected:

The next byte in the above example that follows the Cause Code Origination Point byte, 9C, is the Disconnect Cause Code and is significant in troubleshooting. Use the Disconnect Cause Code file in Resource below to associate a Disconnect Cause Code (in Hex) and the Cause Description to determine the disconnect reason. For the debug isdn q931 command output, drop the highest bit of the cause value before using this table. For example, a cause value of 0x90 becomes 0x10.

| Resources PDF: ISDN Disconnect Casue Code