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Networking Fundamentals · Foundational

Duplex Mismatch

A misconfiguration where one end of a link runs full duplex and the other half duplex, causing poor performance.

Definition

Mismatches usually occur when one side is hard-coded to full duplex and the other autonegotiates, which then falls back to half duplex. The link stays up but suffers late collisions on the half-duplex side and CRC or runt errors on the full-duplex side, especially under load. Fix by setting both sides to auto or both to the same fixed values.

Example

A server hard-set to 100/full connected to a switch port set to auto results in the switch choosing half duplex.

Exam tip

Slow but working link with late collisions and CRC errors = duplex mismatch.

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