If the MAU is a link or fiber type (FOIRL, 10BASE-T, 10BASE-F) then this is equivalent to the link integrity/low light state. For an AUI or coax MAU this indicates whether or not loopback is detected on the DI circuit. The value of this attribute persists between packets for MAU types AUI, 10BASE5, 10BASE2, and 10BASE-FP.

NOTE: IEEE 802.3 modified the following descriptive text at their meeting the week of 3/9/92. New text will be provided at the San Diego IETF plenary.

At power-up or following a reset, the value of this attribute will be unknown for AUI, coax, and 10BASE-FP MAUs. For these MAUs loopback will be tested on each transmission during which no collision is detected. If DI receives input within startUpDelay of the start of output on DO, loopback will be detected. The value of startUpDelay shall be between 15 and 25 BT (bit times). Transition form input to output_idle on DI while output is present on DO may also be detected on loopback. The value of this attribute will only change during non-collided transmissions for AUI, coax, and 10BASE-FP MAUs.