A BERT test is typically used on Serial interfaces which could be T1/E1 or T3/E3 depending on your geographical location. It is intended to test circuit problems and I have not seen it used for ethernet links so far. Regardless Cisco is looking into ways of testing ethernet links as well, like being able to issue a software loopback test on the newer cards. Just keep in mind that for a BERT test you will need a way to loop part of the circuit in order for the test packets to return to you. Normally done with your ISP involved.