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).
 

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

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

[edit]
user@host# edit interfaces interface-name (fastether-options | gigether-options)

[edit interfaces interface-name (fastether-options | gigether-options)]
user@host# set loopback   >>>>>>>>>>> Configure local loopback

To verify the config:

[edit interfaces fe-1/0/0 fastether-options]
user@host# show                
loopback;

Commit the change:

[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.