EXFO ETS-1000L - Manual (Page 19)

Loopback
Ethernet Tester Analyzer 13
Loopback Adjustment

At the Transport layer (L4) the incoming traffic (packets without errors)
is being retransmitted backward with source and destination IP and
MAC addresses swapping and source and destination TCP/UDP
addresses swapping.
Note: Only frames with destination MAC address and destination IP address
corresponding to MAC and IP addresses of the ETS-1000L are retransmitted.
Loopback Adjustment
Connect the ETS-1000L to the Ethernet network and select Loopback layer
by pressing L button. Additional parameters (IP address, MAC address,
etc.) are being configured using the remote management (see Remote
Management on page 15).