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    Interface Diagnostics

    You can use two diagnostic tools to test the physical layer connections of interfaces: loopback testing and bit error rate test (BERT) testing. Loopback testing enables you to verify the connectivity of a circuit. BERT testing enables you to identify poor signal quality on a circuit. This section contains the following topics:

    Configuring Loopback Testing

    Loopback testing allows you to verify the connectivity of a circuit. You can configure any of the following interfaces to execute a loopback test: Aggregated Ethernet, Fast Ethernet, Gigabit Ethernet, E1, E3, NxDS0, serial, SONET/SDH, T1, and T3.

    The physical path of a network data circuit usually consists of segments interconnected by devices that repeat and regenerate the transmission signal. The transmit path on one device connects to the receive path on the next device. If a circuit fault occurs in the form of a line break or a signal corruption, you can isolate the problem by using a loopback test. Loopback tests allow you to isolate segments of the circuit and test them separately.

    To do this, configure a line loopback on one of the routers. Instead of transmitting the signal toward the far-end device, the line loopback sends the signal back to the originating router. If the originating router receives back its own data link layer packets, you have verified that the problem is beyond the originating router. Next, configure a line loopback farther away from the local router. If this originating router does not receive its own data link layer packets, you can assume the problem is on one of the segments between the local router and the remote router’s interface card. In this case, the next troubleshooting step is to configure a line loopback closer to the local router to find the source of the problem.

    There are several types of loopback testing supported by the Junos OS, as follows:

    • DCE local—Loops packets back on the local DCE.
    • DCE remote—Loops packets back on the remote DCE.
    • Local—Useful for troubleshooting physical PIC errors. Configuring local loopback on an interface allows transmission of packets to the channel service unit (CSU) and then to the circuit toward the far-end device. The interface receives its own transmission, which includes data and timing information, on the local router's PIC. The data received from the CSU is ignored. To test a local loopback, issue the show interfaces interface-name command. If PPP keepalives transmitted on the interface are received by the PIC, the Device Flags field contains the output Loop-Detected.
    • Payload—Useful for troubleshooting the physical circuit problems between the local router and the remote router. A payload loopback loops data only (without clocking information) on the remote router’s PIC. With payload loopback, overhead is recalculated.
    • Remote—Useful for troubleshooting the physical circuit problems between the local router and the remote router. A remote loopback loops packets, including both data and timing information, back on the remote router’s interface card. A router at one end of the circuit initiates a remote loopback toward its remote partner. When you configure a remote loopback, the packets received from the physical circuit and CSU are received by the interface. Those packets are then retransmitted by the PIC back toward the CSU and the circuit. This loopback tests all the intermediate transmission segments.

    Table 1 shows the loopback modes supported on the various interface types.

    Table 1: Loopback Modes by Interface Type

    Interface

    Loopback Modes

    Usage Guidelines

    Aggregated Ethernet, Fast Ethernet, Gigabit Ethernet

    Local

    Configuring Ethernet Loopback Capability

    Circuit Emulation E1

    Local and remote

    Configuring E1 Loopback Capability

    Circuit Emulation T1

    Local and remote

    Configuring T1 Loopback Capability

    E1 and E3

    Local and remote

    Configuring E1 Loopback Capability and Configuring E3 Loopback Capability

    NxDS0

    Payload

    Configuring Channelized E1 IQ and IQE Interfaces, Configuring T1 and NxDS0 Interfaces, Configuring Channelized OC12/STM4 IQ and IQE Interfaces (SONET Mode), Configuring Channelized STM1 IQ and IQE Interfaces, and Configuring Channelized T3 IQ Interfaces

    Serial (V.35 and X.21)

    Local and remote

    Configuring Serial Loopback Capability

    Serial (EIA-530)

    DCE local, DCE remote, local, and remote

    Configuring Serial Loopback Capability

    SONET/SDH

    Local and remote

    Configuring SONET/SDH Loopback Capability

    T1 and T3

    Local, payload, and remote

    Configuring T1 Loopback Capability and Configuring T3 Loopback Capability

    See also Configuring the T1 Remote Loopback Response

    To configure loopback testing, include the loopback statement:

    loopback mode;

    You can include this statement at the following hierarchy levels:

    Interface Diagnostics

    BERT allows you to troubleshoot problems by checking the quality of links. You can configure any of the following interfaces to execute a BERT when the interface receives a request to run this test: E1, E3, T1, T3; the channelized DS3, OC3, OC12, and STM1 interfaces; and the channelized DS3 IQ, E1 IQ, and OC12 IQ interfaces.

    A BERT test requires a line loop to be in place on either the transmission devices or the far-end router. The local router generates a known bit pattern and sends it out the transmit path. The received pattern is then verified against the sent pattern. The higher the bit error rate of the received pattern, the worse the noise is on the physical circuit. As you move the position of the line loop increasingly downstream toward the far-end router, you can isolate the troubled portion of the link.

    To configure BERT, you must configure the duration of the test, the bit pattern to send on the transmit path, and the error rate to monitor when the inbound pattern is received.

    To configure the duration of the test, the pattern to send in the bit stream, and the error rate to include in the bit stream, include the bert-period, bert-algorithm, and bert-error-rate statements, respectively, at the [edit interfaces interface-name interface-type-options] hierarchy level:

    [edit interfaces interface-name interface-type-options]bert-algorithm algorithm;bert-error-rate rate;bert-period seconds;

    By default, the BERT period is 10 seconds. You can configure the BERT period to last from 1 through 239 seconds on some PICs and from 1 through 240 seconds on other PICs.

    rate is the bit error rate. This can be an integer from 0 through 7, which corresponds to a bit error rate from 10–0 (1 error per bit) to 10–7 (1 error per 10 million bits).

    algorithm is the pattern to send in the bit stream. For a list of supported algorithms, enter a ? after the bert-algorithm statement; for example:

    [edit interfaces t1-0/0/0 t1-options]
    user@host# set bert-algorithm ?
    Possible completions:
    pseudo-2e11-o152 Pattern is 2^11 -1 (per O.152 standard)
    pseudo-2e15-o151 Pattern is 2^15 - 1 (per O.152 standard)
    pseudo-2e20-o151 Pattern is 2^20 - 1 (per O.151 standard)
    pseudo-2e20-o153 Pattern is 2^20 - 1 (per O.153 standard)
    ...

    For specific hierarchy information, see the individual interface types.

    Note: The 4-port E1 PIC supports only the following algorithms:

      pseudo-2e11-o152     Pattern is 2^11 -1 (per O.152 standard)
      pseudo-2e15-o151     Pattern is 2^15 - 1 (per O.151 standard)
      pseudo-2e20-o151     Pattern is 2^20 - 1 (per O.151 standard)
      pseudo-2e23-o151     Pattern is 2^23 (per O.151 standard)
    

    When you issue the help command from the CLI, all BERT algorithm options are displayed, regardless of the PIC type, and no commit check is available. Unsupported patterns for a PIC type can be viewed in system log messages.

    Note: The 12-port T1/E1 Circuit Emulation (CE) PIC supports only the following algorithms:

      all-ones-repeating   Repeating one bits
      all-zeros-repeating  Repeating zero bits
      alternating-double-ones-zeros  Alternating pairs of ones and zeros
      alternating-ones-zeros  Alternating ones and zeros
      pseudo-2e11-o152     Pattern is 2^11 -1 (per O.152 standard)
      pseudo-2e15-o151     Pattern is 2^15 - 1 (per O.151 standard)
      pseudo-2e20-o151     Pattern is 2^20 - 1 (per O.151 standard)
      pseudo-2e7           Pattern is 2^7 - 1
      pseudo-2e9-o153      Pattern is 2^9 - 1 (per O.153 standard)
      repeating-1-in-4     1 bit in 4 is set
      repeating-1-in-8     1 bit in 8 is set
      repeating-3-in-24    3 bits in 24 are set
    

    When you issue the help command from the CLI, all BERT algorithm options are displayed, regardless of the PIC type, and no commit check is available. Unsupported patterns for a PIC type can be viewed in system log messages.

    Note: The IQE PICs support only the following algorithms:

    all-ones-repeating   Repeating one bits
    all-zeros-repeating  Repeating zero bits
    alternating-double-ones-zeros  Alternating pairs of ones and zeros
    alternating-ones-zeros  Alternating ones and zeros
    pseudo-2e9-o153      Pattern is 2^9  -1 (per O.153 (511 type) standard)
    pseudo-2e11-o152     Pattern is 2^11 -1 (per O.152 and O.153 (2047 type) standards)
    pseudo-2e15-o151     Pattern is 2^15 -1 (per O.151 standard)
    pseudo-2e20-o151     Pattern is 2^20 -1 (per O.151 standard)
    pseudo-2e20-o153     Pattern is 2^20 -1 (per O.153 standard)
    pseudo-2e23-o151     Pattern is 2^23 -1 (per O.151 standard)
    repeating-1-in-4     1 bit in 4 is set
    repeating-1-in-8     1 bit in 8 is set
    repeating-3-in-24    3 bits in 24 are set
    

    When you issue the help command from the CLI, all BERT algorithm options are displayed, regardless of the PIC type, and no commit check is available. Unsupported patterns for a PIC type can be viewed in system log messages.

    Note: BERT is supported on the PDH interfaces of the Channelized SONET/SDH OC3/STM1 (Multi-Rate) MIC with SFP and the DS3/E3 MIC. The following BERT algorithms are supported:

    all-ones-repeating            Repeating one bits
    all-zeros-repeating           Repeating zero bits
    alternating-double-ones-zeros Alternating pairs of ones and zeros
    alternating-ones-zeros        Alternating ones and zeros
    repeating-1-in-4              1 bit in 4 is set
    repeating-1-in-8              1 bit in 8 is set
    repeating-3-in-24             3 bits in 24 are set
    pseudo-2e9-o153               Pattern is 2^9 - 1 (per O.153 standard)
    pseudo-2e11-o152              Pattern is 2^11 - 1 (per O.152 standard)
    pseudo-2e15-o151              Pattern is 2^15 - 1 (per O.151 standard)
    pseudo-2e20-o151              Pattern is 2^20 - 1 (per O.151 standard)
    pseudo-2e20-o153              Pattern is 2^20 - 1 (per O.153 standard)
    pseudo-2e23-o151              Pattern is 2^23 (per O.151 standard)

    Table 2 shows the BERT capabilities for various interface types.

    Table 2: BERT Capabilities by Interface Type

    Interface

    T1 BERT

    T3 BERT

    Comments

    12-port T1/E1 Circuit Emulation

    Yes (ports 0–11)

     
    • Limited algorithms

    4-port Channelized OC3/STM1 Circuit Emulation

    Yes (port 0–3)

     
    • Limited algorithms

    E1 or T1

    Yes (port 0–3)

    Yes (port 0–3)

    • Single port at a time
    • Limited algorithms

    E3 or T3

    Yes (port 0–3)

    Yes (port 0–3)

    • Single port at a time

    Channelized OC12

    N/A

    Yes (channel 0–11)

    • Single channel at a time
    • Limited algorithms
    • No bit count

    Channelized STM1

    Yes (channel 0–62)

    N/A

    • Multiple channels
    • Only one algorithm
    • No error insert
    • No bit count

    Channelized T3 and Multichannel T3

    Yes (channel 0–27)

    Yes (port 0–3 on channel 0)

    • Multiple ports and channels
    • Limited algorithms for T1
    • No error insert for T1
    • No bit count for T1

    These limitations do not apply to channelized IQ interfaces. For information about BERT capabilities on channelized IQ interfaces, see Channelized IQ and IQE Interfaces Properties.

    Starting and Stopping a BERT Test

    Before you can start the BERT test, you must disable the interface. To do this, include the disable statement at the [edit interfaces interface-name] hierarchy level:

    [edit interfaces interface-name]disable;

    After you configure the BERT properties and commit the configuration, begin the test by issuing the test interface interface-name interface-type-bert-start operational mode command:

    user@host> test interface interface-name interface-type-bert-start

    The test runs for the duration you specify with the bert-period statement. If you wish to terminate the test sooner, issue the test interface interface-name interface-type-bert-stop command:

    user@host> test interface interface-name interface-type-bert-stop

    For example:

    user@host> test interface t3-1/2/0 t3-bert-startuser@host> test interface t3-1/2/0 t3-bert-stop

    To view the results of the BERT test, issue the show interfaces extensive | find BERT command:

    user@host> show interfaces interface-name extensive | find BERT

    For more information about running and evaluating the results of the BERT procedure, see the Junos OS Operational Mode Commands.

    Note: To exchange BERT patterns between a local router and a remote router, include the loopback remote statement in the interface configuration at the remote end of the link. From the local router, issue the test interface command.

    Example: Configuring Bit Error Rate Testing

    Configure a BERT test on a T3 interface. In this example, the run duration lasts for 120 seconds. The configured error rate is 0, which corresponds to a bit error rate of 10–0 (1 error per bit). The configured bit pattern of all-ones-repeating means that every bit the interface sends is a set to a value of 1.

    [edit interfaces]t3-1/2/0 {t3-options {bert algorithm all-ones-repeating;bert-error-rate 0;bert-period 120;}}

    Published: 2012-12-11

    Published: 2012-12-11