Configure Interface Diagnostics Tools to Test the Physical Layer Connections

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, and Gigabit Ethernet.

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.

• Configure a line loopback on the local router.

  • The line loopback sends the signal back to the originating router instead of transmitting it to the far-end device.

  • If the originating router receives its own Data Link Layer packets, the problem lies beyond the local router.

• Configure a line loopback farther from the local router - If the originating router does not receive its own Data Link Layer packets, the issue is likely on one of the segments between the local router and the remote router's interface card.

• Narrow down the problem by configuring a line loopback closer to the local router - This helps identify the specific segment causing the issue.

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
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

To configure loopback testing, include the loopback statement:

You can include this statement at the following hierarchy levels:

• [edit interfaces interface-name aggregated-ether-options]

• [edit interfaces interface-name fastether-options]

• [edit interfaces interface-name gigether-options]

• [edit interfaces interface-name serial-options]

To configure BERT:

1. 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.

2. 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).

3. 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:

For specific hierarchy information, see the individual interface types.

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.

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.