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. For backwards compatibility with Ethernet hubs, the standard requires the 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.
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