Autonegotiation and duplex mismatch

The autonegotiation standard does not allow autodetection to detect duplex setting if the other computer is not also set to Autonegotation. When two interfaces are connected and set to different "duplex" modes, the effect of the duplex mismatch is a network that works, but much slower than at its nominal speed. The primary rule for avoiding this is that you must not set one end of a connection to a forced full duplex setting and the other end to autonegotiation.

Many different modes of operations (10BASE-T half duplex, 10BASE-T full duplex, 100BASE-TX half duplex, …) exist for Ethernet over twisted pair cable using 8P8C modular connectors (not to be confused with FCC's RJ45), and most devices are capable of different modes of operations. In 1995, a standard was released for allowing two network interfaces connected to each other to autonegotiate the best possible shared mode of operation. This works well for the case of every device being set to autonegotiate. The autonegotiation standard contained a mechanism for detecting the speed but not the duplex setting of Ethernet peers that did not use autonegotiation.

Interoperability problems lead network administrators to manually set the mode of operation of interfaces on network devices. What would happen is that some device would fail to autonegotiate and therefore had to be set into one setting or another. This often led to duplex setting mismatches: in particular, when two interfaces are connected to each other with one set to autonegotiation and one set to full duplex mode, a duplex mismatch results because the autonegotiation process fails and half duplex is assumed – the interface in full duplex mode then transmits at the same time as receiving, and the interface in half duplex mode then gives up on transmitting a packet. The interface in half duplex mode is not ready to receive a packet, so it signals a clash, and tranmissions are halted, for amounts of time based on backoff (random wait times) algorithms. When both packets start trying to transmit again, they interfere again and the backoff strategy may result in a longer and longer wait time before attempting to transmit again; eventually a transmission succeeds but this then causes the flood and collisions to resume.

Because of the wait times, the effect of a duplex mismatch is a network that is not completely 'broken' but is incredibly slow.