Assuming that the show isdn status command
indicates that communications between the router and the telco’s switch is
performing correctly, the next step is to ensure that the router is actually
dialing the far end.

The most
effective way of troubleshooting dialing is to use the debug
dialer command and then try to establish a call to the remote site.
A common symptom is that the debug dialer command reveals
no dialer activity. This almost always points to one of the following:
- Interesting traffic has not been defined or is incorrect.
- No dialer map is present or the IP address is incorrect.
- The router has no route to the IP address being pinged.
If the debug dialer events command reveals no
dialer activity
, use the
show running-config command to verify that the following
commands are present and are configured with the appropriate parameters:
- Dialer list
- Dialer Map
- Appropriate routing protocol or static route such that the next hop IP
address matches the dialer map
Assuming that the router is actually attempting to dial, Figure
lists a number
of possible error messages and their causes. In the event that dialing occurs
and yet connectivity fails, there are several possible causes including:
- Remote ISDN service line is busy.
- Incorrect or missing ISDN phone number.
- Incorrect encapsulation.
- Incorrect authentication parameters.
The first two problems will prevent a call being established, while the
last two will establish the call only to have it terminated due to Layer 2
errors. To help determine which of these problems is being experienced, try to
ping the remote site. If the line protocol fails to change
to an up state, even briefly, then it is likely that the ISDN call itself is
failing. If the line comes up and then goes down it is more likely that
configuration incompatibilities are terminating the call shortly after it is
established.
If the line protocol goes up and down several times, once
for every echo request, an authentication failure would be a strong suspect.