Performing Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces

 

Checklist for Using Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces

Purpose

To use loopback testing to isolate Fast Ethernet and Gigabit Ethernet interface problems.

Action

Table 1 provides links and commands for using loopback testing for Fast Ethernet and Gigabit Ethernet interfaces.

Table 1: Checklist for Using Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces

Tasks

Command or Action

Diagnose a Suspected Hardware Problem with a Fast Ethernet or Gigabit Ethernet Interface
  1. 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

  1. Verify That the Fast Ethernet or Gigabit Ethernet Interface Is Up

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

  1. 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

  1. Clear Fast Ethernet or Gigabit Ethernet Interface Statistics

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

  1. Ping the Fast Ethernet or Gigabit Ethernet Interface

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

  1. Check for Fast Ethernet or Gigabit 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 or Gigabit Ethernet Interface.

Diagnose a Suspected Hardware Problem with a Fast Ethernet or Gigabit Ethernet Interface

Problem

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

Solution

To diagnose a suspected hardware problem with the Ethernet interface, follow these steps:

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).

  1. Create a Physical Loopback for a Fiber-Optic Interface

  2. Create a Loopback Plug for an RJ-45 Ethernet Interface

  3. Configure a Local Loopback



Create a Physical Loopback for a Fiber-Optic Interface

Action

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

Note

Make sure you use single-mode fiber for a single-mode port and multimode fiber for a multimode port.

Meaning

When you create and then test a physical loopback, you are testing the transmit and receive ports 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.



Create a Loopback Plug for an RJ-45 Ethernet Interface

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

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.

Note

Remember to delete the loopback statement after completing the test.

Verify That the Fast Ethernet or Gigabit Ethernet Interface Is Up

Purpose

Display the status of the Fast Ethernet or Gigabit 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 or Gigabit Ethernet interface is up, use the following Junos OS command-line interface (CLI) operational mode command:

Sample Output

user@host# show interfaces ge-4/0/6 extensive

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 or Gigabit Ethernet Interface Statistics

Purpose

You can reset the Fast Ethernet and Gigabit 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

user@host> clear interfaces statistics ge-4/0/6

Meaning

This command clears the interface statistics counters for the Gigabit Ethernet interface only.

Ping the Fast Ethernet or Gigabit 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 1

user@router> ping 10.108.120.2 bypass-routing interface ge-4/0/6 count 100 rapid

Sample Output 2

user@router> ping 10.108.120.2 bypass-routing interface ge-7/2/1 count 100 rapid

Meaning

In the first sample output, the command sends out 100 ping packets out of the interface to the local IP address. The ping should complete successfully with no packet loss. If there is any persistent packet loss, 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).

The second sample output shows that the time to live (TTL) expired, indicating that the link is receiving the frames from the ping test. The MAC address used is the same as the physical address of the port being tested because this allows the port to accept the frames from the ping test. As the packet is looped over the link, you expect to receive a TLL exceeded message for each ping sent. These messages are generated because the ping packets are repeatedly looped between the router and the physical loopback. When the packet is sent to the other end of the link, which does not exist, the loopback returns the packet back to the same interface, where it is again subjected to the Packet Forwarding Engine fabric for routing. After the route lookup, the TTL is decremented, and the packet is again sent out of the looped interface. This process repeats until the packed is either lost, or the TLL expires with subsequent TTL expired message displayed. Should any errors occur, the packet is discarded and a time-out error is displayed, rather than the expected TTL expired message. Note that the default TTL for ICMP echo packets in Junos OS is 64. This means a given test packet must be successfully sent and received 63 times before a TTL expired message can be generated. You can alter the TTL value to adjust the tolerance for loss, for example, a value of 255 is the most demanding test because now the packet must be sent and received error free 254 times.

Check for Fast Ethernet or Gigabit 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

user@router> show interfaces ge-4/0/6 extensive

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 or Gigabit Ethernet Interface. Keep in mind that 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.