When we use the PHY and MAC Loopback for ETH services test, we most take care if the ETH port of SDH equipment is connected with the data equipment like a switch has other services configured, because the LOOP does a data storm in the data equipment and the services would will be interrupted.
The loops problems does not generated alarms.
It is possible to use the PHY and MAC Loopback for test the service, but it is necessary to disconnect the data equipment with activity service and the ETH port of SDH equipment, and connect other unused date equipment or the meter for test the services.
The loop generate a broadcast storm in the switch.
We can use the function ETHERNET TEST for make a test in a ETH services, it is like a ping and we can counter the packets launched and received between two VCTRUNKs.