Troubleshooting Serial Lines - Internetworking Troubleshooting - page 423

If show isdn status indicates that Layer 1 is DEACTIVATED, then there is a problem with the physical
connectivity on the T1 line. Refer to the previous section “Is the Controller Administratively Down?”
Also verify that the T1 is not administratively down. Use the no shutdown command to bring up the T1
controller.

Step 2

Check whether Layer 2 state is MULTIPLE_FRAME_ESTABLISHED.
The desired Layer 2 State is MULTIPLE_FRAME_ESTABLISHED, which indicates that we are
exchanging Layer 2 frames and have finished Layer 2 initialization.
If Layer 2 is not MULTIPLE_FRAME_ESTABLISHED, use the show controller t1 exec command to
diagnose the problem. Refer to the section “Troubleshooting Using the show controller t1 Command.”
Because show isdn status is a snapshot of the current status, it is possible that Layer 2 is bouncing up
and down despite indicating MULTIPLE_FRAME_ESTABLISHED. Use debug isdn q921 to verify that
Layer 2 is stable.

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
command as necessary.

In a production environment, verify that console logging is disabled. Enter the show
logging command. If logging is enabled, the access server might intermittently freeze up
as soon as the console port gets overloaded with log messages. Enter the no logging
console command.

Note

If debug isdn q921 is turned on and you do not receive any debug outputs, place a call or
reset the controller to get debug outputs.

Note

Verify that Layer 2 is stable. You should observe the debug outputs for messages indicating that the
service is not bouncing up and down. If you see the following types of debug outputs, the line is not
stable:

Step 1

Mar 20 10:06:07.882: %ISDN-6-LAYER2DOWN: Layer 2 for Interface Se0:23, TEI 0 changed
to down
Mar 20 10:06:09.882: %LINK-3-UPDOWN: Interface Serial0:23, changed state to down
Mar 20 10:06:21.274: %DSX1-6-CLOCK_CHANGE: Controller 0 clock is now selected as
clock source
Mar 20 10:06:21.702: %ISDN-6-LAYER2UP: Layer 2 for Interface Se0:23, TEI 0 changed to
up
Mar 20 10:06:22.494: %CONTROLLER-5-UPDOWN: Controller T1 0, changed state to up
Mar 20 10:06:24.494: %LINK-3-UPDOWN: Interface Serial0:23, changed state to up
If Layer 2 does not appear to be stable, refer to the section “Troubleshooting Error Events.”

Verify that you are seeing only SAPI messages in both transmit (TX) and receive (RX) sides. Step 2

Mar 20 10:06:52.505: ISDN Se0:23: TX -> RRf sapi = 0 tei = 0 nr = 0
Mar 20 10:06:52.505: ISDN Se0:23: RX <- RRf sapi = 0 tei = 0 nr = 0
Mar 20 10:07:22.505: ISDN Se0:23: TX -> RRp sapi = 0 tei = 0 nr = 0
Mar 20 10:07:22.509: ISDN Se0:23: RX <- RRp sapi = 0 tei = 0 nr = 0
Mar 20 10:07:22.509: ISDN Se0:23: TX -> RRf sapi = 0 tei = 0 nr = 0
Mar 20 10:07:22.509: ISDN Se0:23: RX <- RRf sapi = 0 tei = 0 nr = 0