Troubleshooting Serial Lines - Internetworking Troubleshooting - page 431

Step 3

Ensure that encapsulation is PPP. If the interface is not using PPP, then use the encapsulation ppp
command in the interface configuration mode to correct it.
maui-nas-03(config-if)# encapsulation ppp

Step 4

Check to see whether loopback is set. Loopback should be set only for testing purposes. Use the no
loopback command to remove loopbacks.
maui-nas-03(config-if)# no loopback

Step 5

Power-cycle the router.

Step 6

If the problem persists, contact your service provider or Cisco TAC.

Troubleshooting a PRI

Whenever troubleshooting a PRI, you need to check whether the E1 is running cleanly on both ends. If
Layer 1 problems have been resolved, as described previously, we must look to Layer 2 and 3 problems.

Troubleshooting Using the show isdn status Command

The show isdn status command is used to display a snapshot of all ISDN interfaces. It displays the status
of Layers 1, 2, and 3.

Verify that Layer 1 is active.
The Layer 1 status should always say ACTIVE unless the E1 is down.
If show isdn status indicates that Layer 1 is DEACTIVATED, then there is a problem with the physical
connectivity on the E1 line. Refer to the section “Is the Controller Administratively Down?”
Also verify that the E1 is not administratively down. Use the no shutdown command to bring up the E1
controller.

Step 1

Check whether Layer 2 state is MULTIPLE_FRAME_ESTABLISHED.
The desired Layer 2 state is MULTIPLE_FRAME_ESTABLISHED, which indicates that the startup
protocol between ISDN switch and end device has been established and that we are exchanging Layer 2
frames.
If Layer 2 is not MULTIPLE_FRAME_ESTABLISHED, use the show controller E1 exec command to
diagnose the problem. Refer to the previous section “Troubleshooting Using the show controller e1
Command,” and the upcoming section “Troubleshooting E1 Error Events.”
Because show isdn status is a snapshot of the current status, it is possible that Layer 2 is bouncing up
and down despite indicating Mulitple_Frame_Established. Use debug isdn q921 to verify that Layer 2
is stable.

Step 2

Using debug q921

The debug isdn q921 command displays data link layer (Layer 2) access procedures that are taking place
at the router on the D-channel.
Ensure that you are configured to view debug messages by using the logging console or terminal
monitor commands, as necessary.