Duplex Mismatch

Duplex mismatch causes collisions/late collisions and leads to poor performance.

Updated: 2026-03-05

Definition

Duplex mismatch happens when one side is full-duplex and the other is half-duplex.

It often causes retransmissions and serious performance issues.

Key points

Classic performance killer on Ethernet
Auto-negotiation consistency matters
Symptoms: errors, collisions, slow transfers

Common mistakes

Hard-setting one side but leaving the other on auto.
Troubleshooting 'application' issues while the real cause is L1/L2.

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Related terms

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