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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


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

Note: Make sure you use single-mode fiber for a single-mode port and multimode fiber for a multimode port for SONET media.


When you create and 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.

Configure a Local Loopback


Because ATM interfaces can be either SONET or T3, you use the sonet-options or t3-options statements to configure a local loopback. Figure 1 illustrates a local loopback configured for an ATM interface.

Figure 1: Local Loopback

Image h1960.gif


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 (sonet-options | t3-options)
  2. Configure the loopback:
    [edit interfaces interface-name (sonet-options | t3-options)]user@host# set loopback local
  3. Verify the configuration:
    user@host# show

    For example:

    [edit interfaces (t3-1/0/0 t3-options)]user@host# show loopback local;
  4. Commit the change:
    user@host# commit

    For example:

    [edit interfaces t3-1/0/0 t3-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.

Note: Remember to delete the loopback statement after completing the test.

Published: 2010-01-29

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