When auto-negotiation is configured the speed is negotiated to the highest speed using the Fast Link Pulses (FLP), and the duplex is also negotiated (full duplex if both supports full duplex, other wise half duplex), however, if auto-negotiation is disabled on either end of the cable, the switch detects the speed anyway based on the incoming electrical signal, but the devices without a configured/negotiated duplex setting must assume a default. Cisco switches use a default duplex setting of half duplex (HDX) (for 10-Mbps and 100-Mbps interfaces) or full duplex (FDX) (for 1000-Mbps interfaces), which can introduced duplex mismatch, example if one side is auto and the other is full, the auto side will assume half duplex, which introduces duplex mismatch.
Accordingly as a best practice, either to make both sides Auto (highest matching speed on both sides, and full duplex if both supports full duplex, other wise half duplex), or configure both sides explicitly to avoid any Duplex mismatch problems.
Comments:Link is established, but the switch does not see any autonegotiation information from NIC. Since Catalyst switches support only full-duplex operation with 1000 Mbps, they default to full-duplex, and this happens only when operating at 1000 Mbps.
3. NIC = 100 / 1000 Mbps, Full-duplex
Switch = 100 / 1000 Mbps, Full-duplex
Correct Manual Configuration
4. NIC = 100 Mbps, Full-duplex [No Link]
Switch = 1000 Mbps, Full-duplex [No Link]
Comment:Neither side establishes link, due to speed mismatch
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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