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| 4.4 |  | Troubleshooting ISDN |
| 4.4.4 |  | Troubleshooting ISDN BRI dialer problems |

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. For more information on troubleshooting PPP authentication refer to the Troubleshooting PPP authentication section. Authentication problems have this characteristic because a failed authentication attempt will immediately terminate the call. This occurs so quickly that next ping packet is able to reinitiate a new call. By contrast, misconfigured parameters such as encapsulation terminate the call only after several keepalives have been missed.