Loopback can be used for many diagnostic purposes. Typically loopback is used to check an IDU, but some model ODUs will allow rf_gen and rf_refl modes which turns the ODU on one side of the link into a reflector and loops the traffic back to the original site. This can be useful to diagnose cable issues as the side in reflector mode is essentially an ODU running without an IDU. (Please note, the IDU is still powering the ODU, so power line noise can still get into the ODU. This test will not completely rule out problems one one side, but may be helpful in identifying them without replacing hardware.)
To use loopback rf_gen & rf_refl, you'll first want to disable your Ethernet ports by unplugging the cables, or disabling the ports in the IDU or your switch. You'll need to do this because the actual traffic will be looped back at the far side. For example:
(trango-config)# port eth ge1 enable off
Port ge1: off
On one side, then do enable loopback with rf_gen. This side will essentially operate as normal, generating rf packets from the IDU.
(trango-config)# loopback rf_gen
Loopback Mode: rf_gen
On the other side of the link, enable rf_refl mode.
(trango-config)# loopback rf_refl
Loopback Mode: rf_refl
This will allow operation of the link without the far side's cable. You can then check the RSSI and MSE in an attempt to identify a potential cable issue. You will likely then want to reverse the roles, the side that is generating will reflect and vice-a-versa. If you identify that the link is good with one side in reflect, but not when it is in generate mode, that may indicate a cable problem on the side that is good only when in reflect mode.
When done with the testing, disable loopback and re-enable the Ethernet ports.
(trango-config)# loopback off
Loopback Mode: off
(trango-config)# port eth ge1 enable on
Port ge1: on