background image
190 Chapter 6: Using ISDN and DDR Technologies
Scenario 6-4 Answers
The answers for this scenario represent a basic troubleshooting guide. By far, they are not the
only answers available.
1
Layer 1 represents the physical layer. Make sure the cabling is properly connected. Ensure
that the proper cables are being used. Make sure you have the proper switch type
configured. Call the telco to ensure the proper setting. Make sure the interface has not been
shutdown. With all cables properly connected and the proper switch type, the layer should
activate.
2
Layer 2 obviously relies on Layer 1. If Layer 1 is not active, Layers 2 and 3 cannot
activate. There are a number of commands useful in troubleshooting Layer 2. Table 6-7
lists some of the privileged EXEC commands:
3
If the TEI is not being negotiated, check the Layer 1 connectivity and the switch type. If
Layer 1 is correctly connected and the switch type is properly configured, there may be a
problem with the installation. If the switch type is correct, and the telco hasn't properly
installed the line, there is no connectivity. Remember, in Europe, a TEI is not assigned
until a call is set up. In North America, there should be on-going communication between
the switch and the router, so the TEI should be assigned. Cisco does not support TEI=0
Example 6-43
Raleigh Router Dial Backup for Load Configuration
interface serial 0
backup load 90 10
Example 6-44
Atlanta Router Dial Backup for Load Configuration
interface serial 0
backup load 90 10
Table 6-8
ISDN Layer 2 Troubleshooting Commands
Command
Explanation
show isdn status
This command is useful in troubleshooting all 3 layers of the ISDN connection.
It shows layer by layer the status of the connection. If Layer 2 is active, you
will see MULTIPLE_FRAME_ESTABLISHED in the output under the Layer 2
information.
debug isdn q921
This command shows the real-time negotiation (or lack thereof) between the
router and the telco switch. TEI negotiation can also be monitored here.
show interface bri0
This command shows a snapshot of the current status (up/down, and so on) of
the interface.