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Configuring Interface Diagnostics Tools to Test the Physical Layer Connections

 

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 that 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.

The following types of loopback testing are supported by Junos OS:

  • DCE local—Loops packets back on the local data circuit-terminating equipment (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 NxDS0 IQ and IQE Interfaces, Configuring T1 and NxDS0 Interfaces, Configuring Channelized OC12/STM4 IQ and IQE Interfaces (SONET Mode), Configuring Fractional E1 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 to Identify a Problem as Internal or External

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:

You can include this statement at the following hierarchy levels:

Configuring BERT Testing

To configure BERT:

  • Configure the duration of the test.

    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. By default, the BERT period is 10 seconds.

  • Configure the error rate to monitor when the inbound pattern is received.

    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).

  • Configure the bit pattern to send on the transmit path.

    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:

    user@host# set bert-algorithm ?

For specific hierarchy information, see the individual interface types.

Note

The four-port E1 PIC supports only the following algorithms:

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:

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:

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:

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

Yes (channel 0–11)

  • Single channel at a time

  • Limited algorithms

  • No bit count

Channelized STM1

Yes (channel 0–62)

  • 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:

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:

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

For example:

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

For more information about running and evaluating the results of the BERT procedure, see the CLI Explorer.

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.