Abstract: The invention is test apparatus and methods for performing loopback tests. The tests involve generating a packet-based test message having a source address and a destination address. When the test message arrives at the destination, the addresses are exchanged and the message is returned to the source. The addresses can be level 2 addresses, such as MAC addresses, and/or level 3 addresses, such as IP addresses. The returned message and the propagation properties observed for the test message can be used to determine information about the network being tested.