Cisco Loopback Tests

Loopback Modes

There are three main loopback modes as follows: local, network (both line and payload), and remote (line and payload). Specify the loopback format using the loopback [local | network | remote] command.

Examples of specific loopback modes follow:

  • Set the first T1 into local loopback as follows:

    Router# config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# interface serial 3/0:1
Router(config-if)# loopback local
    In the preceding example, local loopback loops the router output data back toward the router at the T1 framer and sends an AIS signal out toward the network.

  • Set the first T1 into network line loopback as follows:

    Router# config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# interface serial 3/0:1
Router(config-if)# loopback network line
    In the preceding example, network line loopback loops the data back toward the network (before the T1 framer).

  • Set the first T1 into network payload loopback as follows:

    Router# config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# interface serial 3/0:1
Router(config-if)# loopback network payload
    In the preceding example, network payload loopback loops just the payload data back toward the network at the T1 framer.

  • The syntax of the loopback remote command follows:

    loopback [remote {line {fdl ansi | inband} | payload fdl ansi}]
    Set the first T1 into remote line inband loopback, as follows:
    Router# config t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)# int serial 3/0:1
Router(config-if)# loop remote line inband
    In the preceding example, remote line inband loopback sends a repeating 5-bit inband pattern (of 00001) to the remote end requesting that it enter into a network line loopback.

    Set the first T1 into remote line FDL ANSI bit loopback, as follows:
    Router# config t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)# int serial 3/0:1
Router(config-if)# loop remote line fdl ansi
    In the preceding example, remote line FDL ANSI bit loopback sends a repeating 16-bit ESF data link code word (of 00001110 11111111) to the remote end requesting that it enter into a network line loopback.

    Set the first T1 into remote payload FDL ANSI bit loopback, as follows:
    Router# config t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)# int serial 3/0:1
Router(config-if)# loop remote payload fdl ansi
    In the preceding example, remote payload ANSI bit loopback sends a repeating 16-bit ESF data link code word (of 00010100 11111111) to the remote end requesting that it enter into a network payload loopback.

Checking Bit Errors Using a BERT

To check bit errors using a bit error test (BERT), use the following command: 

[no] t1 t1 channel bert pattern {2^15 | 2^20 | 2^23 | 0s | 
1s} interval minutes

where t1 channel is 1-28, 2^15 is an exponential number that represents a pseudo-random repeating pattern that is 32767 bits long, 2^20 is a pseudo-random repeating pattern that is 1048575 bits long, 2^23 is a pseudo-random repeating pattern that is 8388607 bits long, 0s is a pattern of all zeroes (00000000...), 1s is a pattern of all 1s (111111...), and minutes are 1-14400, which designate the time the BERT will run. To stop the BERT, use the optional no form of the command. Following is an example of this command, where the pseudo-random pattern 2^20 is sent and repeats on the first T1 channel for 60 minutes:

Router# t1 1 bert pattern 2^20 interval 60

Note that the BERT command is not saved in NVRAM. The test patterns from the PA-CT3/4T1 port adapter are framed test patterns; therefore, they are inserted into the payload of a framed T1 signal.

To display the BERT results, use the following EXEC commands: sh cont t3 number or sh cont t3 number brief. Following are examples of displaying BERT results output:

  1. Display BERT output results during a test, as follows:

    Router# sh cont t3 5/0 b
T3 5/0 is up.
CT3 H/W Version : 4, CT3 ROM Version : 0.116, CT3 F/W Version : 20.2.0
  Mx H/W version : 2, Mx ucode ver : 1.25
  Applique type is Channelized T3
  No alarms detected.
  FEAC code received : No code is being received
  Framing is M23, Line Code is B3ZS, Clock Source is Internal.
  T1 1 is down, speed: 1536 kbs, non-inverted data
  timeslots: 1-24
  FDL per ANSI T1.403 and AT&T 54016 spec.
  Configured for FDL Remotely Line Looped
  No alarms detected.
  Framing is ESF, Line Code is B8ZS, Clock Source is Internal.
  BERT test result (running)
      Test Pattern : All 0's, Status : Sync, Sync Detected : 1
      Interval : 4 minute(s), Time Remain : 4 minute(s)
      Bit Errors(Sync BERT Started) : 0 bits
      Bit Errors(Sync last Sync) : 0 bits , Bits Received : 7 Mbits

    Note When the T1 is under BERT, its line state is down. When under BERT (in the running state), the "Total Bit Errors" value is not valid if the "Status" field is "Not Sync."

  2. When the test is done, display BERT results output as follows:

    Router# sh cont t3 5/0 b
T3 5/0 is up.
  CT3 H/W Version : 4, CT3 ROM Version : 0.116, CT3 F/W Version : 20.2.0
  Mx H/W version : 2, Mx ucode ver : 1.25
  Applique type is Channelized T3
  No alarms detected.
  FEAC code received : No code is being received
  Framing is M23, Line Code is B3ZS, Clock Source is Internal.
  T1 1 is up, speed: 1536 kbs, non-inverted data
  timeslots: 1-24
  FDL per ANSI T1.403 and AT&T 54016 spec.
  Configured for FDL Remotely Line Looped
  No alarms detected.
  Framing is ESF, Line Code is B8ZS, Clock Source is Internal.
  BERT test result (done)
      Test Pattern : All 0's, Status : Not Sync, Sync Detected : 1
      Interval : 4 minute(s), Time Remain : 0 minute(s)
      Bit Errors(Sync BERT Started) : 0 bits
      Bit Errors(Sync last Sync) : 0 bits , Bits Received : 368 Mbits

    Note The "Status" field is irrelevant when the BERT is done. If the "Sync Detected" counter is 0, the Bit Errors fields have no meaning.

  3. When the test is stopped prematurely, display BERT results output as follows:

    Router# sh cont t3 5/0 b
T3 5/0 is up.
  CT3 H/W Version : 4, CT3 ROM Version : 0.116, CT3 F/W Version : 20.2.0
  Mx H/W version : 2, Mx ucode ver : 1.25
  Applique type is Channelized T3
  No alarms detected.
  FEAC code received : No code is being received
  Framing is M23, Line Code is B3ZS, Clock Source is Internal.
  T1 1 is up, speed: 1536 kbs, non-inverted data
  timeslots: 1-24
  FDL per ANSI T1.403 and AT&T 54016 spec.
  Configured for FDL Remotely Line Looped
  No alarms detected.
  Framing is ESF, Line Code is B8ZS, Clock Source is Internal.
  BERT test result (done)
      Test Pattern : All 0's, Status : Not Sync, Sync Detected : 1
      Interval : 4 minute(s), Time Remain : 2 minute(s) (unable to complete)
      Bit Errors(Sync BERT Started) : 0 bits
      Bit Errors(Sync last Sync) : 0 bits , Bits Received : 368 Mbits

    Note In the preceding three examples, the "Sync Detected" counter indicates the number of times the pattern sync is detected (from No Sync to Sync), the "Bit Errors(Sync BERT Started)" counter indicates the number of bit errors during BERT, and the "Bit Errors(Sync last Sync)" counter shows the number of bit errors since the last pattern sync is detected.