The IEEE 802.1ab ETH-LB is unicast message used to check whether a fault occurs on the link between the local and peer terminal equipment. On receiving the loopback message, the endpoint checks whether conditions are met. If so, the receive node returns a loopback reply (LBR) packet (another unicast MAC message) to the originating device. The source node reports an alarm if it does not receive any LBR within five seconds after sending the LBM. As intermediate nodes perform only Layer 2 forwarding, LBMs can carry additional data that must be returned in LBRs without change. LBMs can therefore also be used for traffic test purposes.
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