EXFO ETS-1000L - Manual (Page 18)

Loopback
12 ETS-1000L

At the Data link layer (L2), the incoming traffic (frames without errors)
is being retransmitted backward with swapping destination and source
MAC addresses.
Note: Frames with destination MAC address different than MAC address of
ETS-1000L are not retransmitted.
Note: Frames with equal destination and source MAC address are not
retransmitted at the data link, network and transport layers.

At the Network layer (L3) the incoming traffic (packets without errors)
is being retransmitted backward with source and destination IP and
MAC 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.