ADTRAN 1200130L2 - Manual (Page 118)

104 FSU User Manual 61200.130L1-1
Chapter 11. Diagnostics
Figure 11-3
VT 100 Ping Menu
Loopback All Ports (LOOPBACK ENA)
This selection places each port (including the network port) into
loopback mode. A loopback is an intrusive test used for problem
isolation.
Disable Loopbacks (LOOPBACK DIS)
This selection disables active loopbacks tests, returning the unit
to normal mode.