Technical Documentation

Create a Loopback

Purpose

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

Image h1837.gif

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:
    [edit]user@host# edit interfaces interface-name (fastether-options | gigether-options)
  2. Configure the local loopback:
    [edit interfaces interface-name (fastether-options | gigether-options)] user@host# set loopback
  3. Verify the configuration:
    user@host# show

    For example:

    [edit interfaces fe-1/0/0 fastether-options]user@host# show loopback;
  4. Commit the change:
    user@host# commit

    For example:

    [edit interfaces fe-1/0/0 fastether-options]user@host# commit commit complete

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.


Published: 2010-01-29