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In Ethernet, a duplex mismatch is a condition where two connected devices operate in different duplex modes, that is, one operates in half duplex while the other one operates in full duplex. The effect of a duplex mismatch is a network that works but is often much slower than its nominal speed. Duplex mismatch may be caused by manually setting two connected network interfaces at different duplex modes or by connecting a device that performs autonegotiation to one that is manually set to a full duplex mode. Duplex mismatch due to autonegotiation When a device set to autonegotiation is connected to a device that is not using autonegotiation, the autonegotiation process fails. The autonegotiating end of the connection is still able to correctly detect the speed of the other end, but cannot correct the duplex mode. The standard requires the use of half duplex in these conditions. Therefore, the autonegotiating end of the connection uses half duplex while its peer is locked at full duplex, and this is a duplex mismatch. The Ethernet standards body, IEEE 802.3, recommend enabling autonegotiation on all connections. Network equipment allows autonegotiation to be disabled and on some networks, autonegotiation is disabled on all ports and a fixed modality of 100 Mbit/s and full duplex is used. This was especially done by network administrators upon the introduction of autonegotiation, because of interoperability issues with the initial autonegotiation specification. A fixed mode of operation works if both ends of a connection are locked to the same settings. However, maintaining such a network and guaranteeing consistency is difficult. Since autonegotiation is generally the manufacturer’s default setting it is almost certain that, in an environment where the policy is to fix port settings, sooner or later someone will leave a port set to use autonegotiation by mistake.