Duplex mismatch causes severe performance problems. One end on full duplex sends constantly. The other on half duplex sees this as collisions. You get to percent of expected throughput. Symptoms: late collisions, FCS errors, slow transfers. Fix: set both ends to auto-negotiate, or manually configure identical settings. Never set one end to auto and the other to fixed.
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$ curl repovive.com/roadmaps/network-design/physical-and-data-link-fundamentals/speed-and-duplex-troubleshooting
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