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Cisco T1 PRI Troubleshooting

Contents

Introduction

Using the show isdn status Command

Using the debug q921 Command

Introduction

When troubleshooting a Primary Rate Interface (PRI), ensure that the T1
is running properly on both ends. If Layer 1 problems have been resolved,
look for problems on Layers 2 and 3. Use the show controller t1
command to verify that the configuration of the line matches that of the
remote end. Ensure that the framing, line coding, and clock source are
configured correctly.

Using the show isdn status Command

The show isdn status command displays a summary of all ISDN interfaces.
It also displays the status of Layers 1, 2, and 3. Complete the following
steps to check the status of the layers:

  1. Verify that Layer 1 is in the ACTIVE state. The status of Layer 1 should
    always be ACTIVE unless the T1 is down.


    2. If the show isdn status command output indicates that Layer
    1 is DEACTIVATED, then there is a problem with the physical connectivity
    of the T1 line. If the line is administratively down, use the no shutdowncommand to restart the interface.

  1. Ensure that Layer 2 is in the MULTIPLE_FRAME_ESTABLISHED state. This is
    the desired state for Layer 2, indicating that Layer 2 frames are being
    exchanged and Layer 2 initialization has finished.


    2. If Layer 2 is not in the MULTIPLE_FRAME_ESTABLISHED state, use the
    show controller t1     EXEC command to diagnose the problem. For more information,
    see the T1
    Alarm Troubleshooting     document.

    Since the show isdn status command displays a summary of the
    current status, it is possible that Layer 2 is bouncing up and down despite
    indicating a MULTIPLE_FRAME_ESTABLISHED state. Use the debug isdn q921
    command to verify that Layer 2 is stable.

    Following is an example of show isdn status output:

      maui-nas-03#show isdn status Global ISDN Switchtype = primary-5ess
      ISDN Serial0:23 interface dsl 0, interface ISDN Switchtype = primary-5essLayer 1 Status: ACTIVE Layer 2 Status: TEI = 0, Ces = 1, SAPI =
      0, State = MULTIPLE_FRAME_ESTABLISHED Layer 3 Status: 5 Active Layer
      3 Call(s) Activated dsl 0 CCBs = 5 CCB:callid=7D5, sapi=0, ces=0, B-chan=9,
      calltype=DATA CCB:callid=7D6, sapi=0, ces=0, B-chan=10, calltype=DATA CCB:callid=7DA,
      sapi=0, ces=0, B-chan=11, calltype=DATA CCB:callid=7DE, sapi=0, ces=0,
      B-chan=1, calltype=DATA CCB:callid=7DF, sapi=0, ces=0, B-chan=2, calltype=DATA
      The Free Channel Mask: 0x807FF8FC ISDN Serial1:23 interface dsl
      1, interface ISDN Switchtype = primary-5ess Layer 1 Status: ACTIVE
      Layer 2 Status: TEI = 0, Ces = 1, SAPI = 0, State = TEI_ASSIGNED
      Layer
      3 Status: 0 Active Layer 3 Call(s) Activated dsl 1 CCBs = 0 The Free Channel
      Mask: 0x807FFFFF Total Allocated ISDN CCBs = 5

    Notice that T1 0 (whose D channel is Serial 0:23) has Layer 1 as ACTIVE
    and Layer 2 as MULTIPLE_FRAME_ESTABLISHED indicating that the signaling
    channel is functioning correctly and is exchanging Layer 2 frames with
    the telco switch. The D channel (Serial1:23) for T1 1 has Layer 1 ACTIVE,
    but Layer 2 is TEI_ASSIGNED. This indicates that the PRI is not exchanging
    Layer 2 frames with the switch. Use the show controller t1 x
    command to troubleshoot. Refer to the T1
    Troubleshooting     flowchart for more information

Using the debug q921 Command

The debug isdn q921 command displays data link layer (Layer 2) access
procedures that are occurring at the router on the D-channel.

Ensure you are configured to view debug messages by using the logging
console or terminal monitor command.

Note: In a production environment, verify that console logging
is disabled by using the show logging command. If logging is enabled,
the access server might intermittently stop working when the console port
is overloaded with log messages. Enter the no logging console command
to disable logging.

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.

Complete the following steps to ensure that the data link layer access
procedures are occurring at the router on the D-channel:

  1. Verify that Layer 2 is stable by looking for messages in the debug output.
    If the line is bouncing up and down, output similar to the following will
    appear:
    2. 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 downMar 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
  2. Verify that only service access point identifier (SAPI) messages appear
    on both the transmit (TX) and receive (RX) sides. For example:
    3. 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
  3. Verify that asynchronous balanced mode extended (SABME) messages do not
    appear. These messages indicates that Layer 2 is trying to reinitialize.
    The messages usually appear when poll requests (RRp) are transmitted and
    there is no response from the switch (RRf), or vice versa. Following are
    examples of SABME messages:
    4. Mar 20 10:06:21.702: ISDN Se0:23: RX <- SABMEp sapi = 0 tei = 0
Mar 20 10:06:22.494: ISDN Se0:23: TX -> SABMEp sapi = 0 tei = 0

    If SABME messages appear, complete the following steps:

    1. Use the show running-config command to ensure that isdn switch-type
      and pri-group timeslots are configured correctly. Contact your Service
      Provider for the correct values.
    2. To change the isdn switch-type and pri-group settings, enter
      the following commands:
      3. maui-nas-03#configure terminal
maui-nas-03(config)#isdn switch-type primary-5ess
maui-nas-03(config)#controller t1 0
maui-nas-03(config-controlle)#pri-group timeslots 1-24
  4. Ensure the D-channel is up using the show interfaces serial number:23
    command, where the number is the interface number.


    5. If the D-channel is not up, use the no shutdown command to
    bring it up. For example:

    maui-nas-03(config)#interface serial 0:23
maui-nas-03(config-if)#no shutdown
  5. Ensure encapsulation is PPP. If not, use the encapsulation ppp command
    to set encapsulation. For example:
    6. maui-nas-03(config-if)#encapsulation ppp
  6. Ensure the interface is in loopback mode. Loopback should be set only for
    testing purposes. Use the no loopback command to remove loopbacks.
    For example:
    7. maui-nas-03(config-if)#no loopback
  7. Power cycle the router.