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Networking Fundamentals · Foundational

Autonegotiation

An Ethernet mechanism through which connected devices exchange capabilities and select the highest common speed and duplex.

Definition

Autonegotiation (IEEE 802.3 Clause 28) uses fast link pulses on copper and is mandatory for 1000BASE-T. If one side is fixed, the other can usually detect speed but not duplex, defaulting to half duplex at 10/100 Mbps.

Example

Two NICs supporting 10/100/1000 negotiate to 1000 Mbps full duplex.

Exam tip

Best practice is auto on both ends or hard-coded on both ends, never mixed.

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