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

Figure 21: 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:

   [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.