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Duplex mismatch
On an Ethernet connection, 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 link that operates inefficiently. 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.〔(【引用サイトリンク】title=Switch Port Duplex Mismatch )〕
== 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 correctly detect the duplex mode. For backwards compatibility with Ethernet hubs, the standard requires the autonegotiating device to use of half duplex in these conditions. Therefore, the autonegotiating end of the connection uses half duplex while the non-negotiating peer is locked at full duplex, and this is a duplex mismatch.
The Ethernet standards and major Ethernet equipment manufacturers recommend enabling autonegotiation. Nevertheless 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. That was often done by network administrators intentionally upon the introduction of autonegotiation, because of interoperability issues with the initial autonegotiation specification. The fixed mode of operation works well 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 have fixed port settings, someone will sooner or later leave a port set to use autonegotiation by mistake.〔(【引用サイトリンク】title=Autonegotiation on Ethernet – It Works, It Should Be Mandatory! )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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