Troubleshooting ISDN Connections - Internetworking Troubleshooting - page 484

• Does the call complete?
• Is data passing across the link?
• Is the session established? (PPP or terminal)
For modem connections, a data call looks the same as a terminal session coming in until the end, when
the data call goes to negotiate PPP.
For incoming calls involving digital modems, first make sure that the underlying ISDN or CAS is
receiving the call. If you are using an external modem, you can skip the ISDN and CAS group sections.

Incoming ISDN Call Troubleshooting

Use the command debug isdn q931 to watch the q931 signaling messages go back and forth while the
router negotiates the ISDN connection. Here's an example output from a successful connection:
Router# debug isdn q931
RX <- SETUP pd = 8 callref = 0x06
Bearer Capability i = 0x8890
Channel ID i = 0x89
Calling Party Number i = 0x0083, `5551234'
TX -> CONNECT pd = 8 callref = 0x86
RX <- CONNECT_ACK pd = 8 callref = 0x06

The SETUP message indicates that a connection is being initiated by the remote end. The call reference
numbers are maintained as a pair. In this case, the call reference number for the incoming side of the
connection is 0x06, while the call reference number of the outbound side of the connection is 0x86. The
bearer capability (often referred to as the bearercap) tells the router what kind of call is coming in. In
this case, the connection is type 0x8890. That value indicates “ISDN speed 64 kbps.” If the bearercap
had been 0x8090A2, it would have indicated “Speech/voice call u-law.”
If no setup message was seen coming in, verify the correct number (try calling it manually, if it is
voice-provisioned) and check the status of the ISDN interface (see Chapter 16, “Troubleshooting Dialup
Connections” ). If all that checks out, make sure that the call originator is making the correct call. Contact
the telco to trace the call to see where it's being sent. If the connection is a long-distance one, try a
different long-distance carrier using a 1010 long-distance code.
If the call coming in is an async modem call, make sure that the line is provisioned to allow voice calls.

BRI async modem calling is a feature of 3600 routers running 12.0(3)T or later. It requires
a recent hardware revision of the BRI interface network module. WIC modules do not
support async modem calling.

Note

If the call arrived but did not complete, look for a cause code (see Table 17-10). A successful completion
is shown by a connect-ack being issued.
If this is an async modem call, move forward to the “Incoming Modem Call Troubleshooting” section.
At this point, the ISDN call is connected, but no data has been seen coming across the link. Use the
command debug ppp negotiate to see whether any PPP traffic is coming across the line. If not, there
may be a speed mismatch. To determine whether this is the case, use the show running-config privileged
exec command to view the router configuration. Check the dialer map interface configuration command
entries in the local and remote router. These entries should look similar to the following:

dialer map ip 131.108.2.5 speed 56 name C4000