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Perform Loopback Test for Fast Ethernet Interfaces

SUMMARY Learn how to effectively perform loopback testing for Fast Ethernet interfaces. This guide provides a comprehensive checklist for diagnosing hardware and circuit problems, creating and verifying loopbacks, configuring ARP entries, clearing interface statistics, and troubleshooting through pings and error checks.

Checklist to Use Loopback Test for Fast Ethernet Interfaces

Purpose

To use loopback testing to isolate Fast Ethernet Interface problems.

Action

The table below provides links and commands for using loopback testing for Fast Ethernet Interfaces.

Table 1: Checklist to Use Loopback Test for Fast Ethernet Interfaces

Tasks

Command or Action

Diagnose a Suspected Hardware Problem with a Fast Ethernet Interface

Create a Loopback

 

Connect the transmit port to the receive port.

Cross pin 1 (TX+) and pin 3 (RX+) together, and pin 2 (TX-) and pin 6 (RX-) together.

[edit interfaces interface-name (fastether-options | gigether-options)] set loopback show commit

Verify That the Fast Ethernet Interface Is Up

show interfaces (fe-fpc/pic/port | ge-fpc/pic/port)

Configure a Static Address Resolution Protocol Table Entry

show interfaces ge-fpc/pic/port [edit interfaces interface-name unit logical-unit-number family inet address address] set arp ip-address mac mac-address show commit run show arp no-resolve

Clear Fast Ethernet Interface Statistics

clear interfaces statistics fe-fpc/pic/port | ge-fpc/pic/port

Ping the Fast Ethernet Interface

ping remote-IP-address bypass-routing interface (fe-fpc/pic/port | ge-fpc/pic/port count 100 rapid

Check for Fast Ethernet Interface Error Statistics

show interfaces (fe-fpc/pic/port | ge-fpc/pic/port ) extensive

Diagnose a Suspected Circuit Problem

Perform Steps 2 through 8 from Diagnose a Suspected Hardware Problem with a Fast Ethernet Interface.

Diagnose a Suspected Hardware Problem with a Fast Ethernet Interface

Problem

When you suspect a hardware problem, take the following steps to help verify if there is a problem.

Create a Loopback

You can create a physical loopback or configure a local loopback to help diagnose a suspected hardware problem. Creating a physical loopback is recommended because it allows you to test and verify the transmit and receive ports. If a field engineer is not available to create the physical loopback, you can configure a local loopback for the interface. The local loopback creates a loopback internally in the Physical Interface Card (PIC).

Create a Physical Loopback for a Fiber-Optic Interface

Purpose

To verify the functionality of the fiber-optic interface by testing both the transmit and receive ports, ensuring that the PIC (Physical Interface Card) is working correctly. This test is typically performed by a field engineer to confirm the integrity of the connection.

Action

To create a physical loopback at the port, connect the transmit port to the receive port using a known good fiber cable.

Use single-mode fiber for a single-mode port and multimode fiber for a multimode port.

Meaning

When you create and test a physical loopback, you are effectively testing the transmit and receive capabilities of the PIC. This procedure provides a comprehensive test of the PIC, confirming that both ports are operational and the fiber-optic connection is functioning as expected.

Create a Loopback Plug for an RJ-45 Ethernet Interface

Purpose

To verify the functionality of an RJ-45 Ethernet interface by creating a physical loopback plug that allows for the testing of the interface's transmit and receive capabilities.

Action

To create a loopback plug, cross pin 1 (TX+) and pin 3 (RX+) together, and cross pin 2 (TX-) and pin 6 (RX-) together. You need the following equipment to create the loopback:

  • A 6-inch long CAT5 cable

  • An RJ-45 connector

  • A crimping tool

Figure 1 illustrates how to create a loopback plug for an RJ-45 Ethernet interface.

Figure 1: RJ-45 Ethernet Loopback Plug RJ-45 Ethernet Loopback Plug

Meaning

When you create and then test a physical loopback, you are testing the RJ-45 interface of the PIC. This action is recommended if a field engineer is available to create the physical loop as it provides a more complete test of the PIC.

Configure a Local Loopback

Purpose

To configure a local loopback on an Ethernet interface for testing the internal functionality of the PIC without needing a physical connection between the transmit and receive ports.

Action

To configure a local loopback without physically connecting the transmit port to the receive port, follow these steps:

  1. In configuration mode, go to the following hierarchy level:

  2. Configure the local loopback:

  3. Verify the configuration:

    For example:

  4. Commit the change:

    For example:

When you create a local loopback, you create an internal loop on the interface being tested. A local loopback loops the traffic internally on that PIC. A local loopback tests the interconnection of the PIC but does not test the transmit and receive ports. On an Ethernet interface, you cannot create a remote loopback, therefore there is no option to use a local or remote statement. Simply including the loopback statement at the [edit interfaces interface-name (fastether-options | gigether-options] hierarchy level, places the interface into local loopback mode.

Delete the loopback statement after completing the test.

Meaning

Configuring a local loopback internally loops traffic on the interface, verifying the PIC's internal connections. The show command will display the loopback statement in the interface configuration, confirming that the interface is in loopback mode. Remember to delete the loopback configuration after testing

Verify That the Fast Ethernet Interface Is Up

Purpose

Display the status of the Fast Ethernet Interface to provide the information you need to determine whether the physical link is up or down.

Action

To verify that the status of the Fast Ethernet Interface is up, use the following Junos OS command-line interface (CLI) operational mode command:

Sample Output

Meaning

The sample output shows that the link is up and there are no alarms in this loopback configuration. When an internal loopback is configured, the physical loopback should come up without an alarm.

Sample Output

When you see that the physical link is down, there may be a problem with the port. The following output is an example of the show interfaces fe-fpc/pic/port command when the physical link is down:

Meaning

The sample output shows that the physical link is down and there are active alarms and defects.

Table 2 presents problem situations and actions for a physical link that is down.

Table 2: Problems and Solutions for a Physical Link That Is Down

Problem

Action

Cable mismatch

Verify that the fiber connection is correct.

Damaged and/or dirty cable

Verify that the fiber can successfully loop a known good port of the same type.

Too much or too little optical attenuation

Verify that the attenuation is correct per the PIC optical specifications.

The transmit port is not transmitting within the dBm optical range per the specifications

Verify that the Tx power of the optics is within range of the PIC optical specification.

Mismatch between the cable type and the port

Verify that a single-mode fiber cable is connected to a single-mode interface and that a multimode fiber cable is connected to a multimode interface. (This problem does not always cause the physical link to go down; errors and dropped packets are sometimes the result.)

Configure a Static Address Resolution Protocol Table Entry

Purpose

Configure a static Address Resolution Protocol (ARP) entry to allow a packet to be sent out of a looped Ethernet interface.

Note:

Remove the static ARP entry at the end of the loop test after you have completed the tests and monitored interface traffic.

Action

To configure a static ARP table entry for a Gigabit Ethernet interface, follow these steps. You can follow the same procedure to configure a static ARP entry for a Fast Ethernet interface.

  1. Find the Media Access Control (MAC) address for the Gigabit Ethernet interface:

    user@host> show interfaces ge-fpc/pic/port

  2. In configuration mode, go to the following hierarchy level:

  3. Configure the static ARP entry:

  4. Commit the configuration:

  5. Verify that the static ARP entry is installed:

Meaning

The sample output is for Step 1 through Step 6 and shows that a static ARP entry was configured on Gigabit Ethernet interface ge-4/0/6.

Clear Fast Ethernet Interface Statistics

Purpose

You can reset the Fast Ethernet Interface statistics. Resetting the statistics provides a clean start so that previous input/output errors and packet statistics do not interfere with the current diagnostics.

Action

To clear all statistics for the interface, use the following Junos OS CLI operational mode command:

Sample Output

Meaning

This command clears the interface statistics counters for the Fast Ethernet Interface only.

Ping the Fast Ethernet Interface

Purpose

Use the ping command to verify the loopback connection.

Action

To send ping packets from the Ethernet interface, use the following Junos OS CLI operational mode command:

Sample Output

Meaning

  • The sample output shows that the time to live (TTL) expired, indicating that the link is receiving frames from the ping test.

  • The MAC address used matches the physical address of the port being tested, allowing the port to accept the frames from the ping test.

  • As the packet loops over the link, a "TTL exceeded" message is expected for each ping sent.

  • These messages are generated because the ping packets repeatedly loop between the router and the physical loopback.

  • When the packet reaches the other end of the link (which doesn't exist), the loopback sends it back to the same interface, where the Packet Forwarding Engine fabric routes it again.

  • After each route lookup, the TTL decreases, and the packet is sent out through the looped interface until the TTL expires, triggering the "TTL expired" message.

  • If any errors occur, the packet is discarded, and a time-out error is displayed instead of the TTL expired message.

  • By default, the TTL for ICMP echo packets in Junos OS is 64, meaning the packet must loop 63 times before a "TTL expired" message is generated.

  • You can adjust the TTL value to change the test's tolerance for loss. For example, a TTL of 255 requires the packet to loop 254 times without error for a "TTL expired" message.

Check for Fast Ethernet Interface Error Statistics

Purpose

Persistent interface error statistics indicate that you need to open a case with the Juniper Networks Technical Assistance Center (JTAC).

Action

To check the local interface for error statistics, use the following Junos OS CLI operational mode command:

Sample Output

Meaning

Check for any error statistics. There should not be any input or output errors. If there are any persistent input or output errors, open a case with the Juniper Networks Technical Assistance Center (JTAC) at support@juniper.net, or at 1-888-314-JTAC (within the United States) or 1-408-745-9500 (from outside the United States).

Diagnose a Suspected Circuit Problem

Purpose

When you suspect a circuit problem, it is important to work with the transport-layer engineer to resolve the problem. The transport-layer engineer may create a loop to the router from various points in the network. You can then perform tests to verify the connection from the router to that loopback in the network.

Action

After the transport-layer engineer has created the loop to the router from the network, you must verify the connection from the router to the loopback in the network. Follow Step 2 through Step 8 in Diagnose a Suspected Hardware Problem with a Fast Ethernet Interface. Any problems encountered in the test indicate a problem with the connection from the router to the loopback in the network.

By performing tests to loopbacks at various points in the network, you can isolate the source of the problem.