Troubleshooting ISDN
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 in Figure defines the Cause Code Origination Points to help interpret where the call was disconnected.

The next byte that follows the Cause Code Origination Point byte is the Disconnect Cause Code and is significant in troubleshooting. In the example above, the Disconnect Cause Code is 9C. The descriptions for the Disconnect Cause Codes can be found at the weblink below. The Disconnect Cause Code 9C indicates that the connection failed because of an invalid number format. This means that the destination address is presented in an unrecognizable format, or the destination address is incomplete.


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