If a switch port is configured with 100Mbps and Full Duplex (auto-negotiation is disabled -> no FLPs are sent), but the Workstation's NIC is configured for auto-negotiation --> Without auto-negotiation on the switch, FLPs (Fast Link Pulses) will not be sent to the workstation, and as a result, the workstation will configure itself to half-duplex (FLP was used to negotiate both speed and duplex), which can result in Duplex mismatch.
Switches detect duplex settings through auto-negotiation only. If both ends have auto-negotiation enabled, the duplex is negotiated. However, if either device on the cable disables auto-negotiation, the devices without a configured 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 -> Always Full can't be Half).
Duplex mismatch is a situation in which the switch is operating at full-duplex and the connected device is operating at half-duplex, or the other way around. The result of a duplex mismatch is extremely slow performance, intermittent connectivity, and loss of connection.
NOTE: If auto-negotiation is disabled on either end of the cable, the switch detects the speed anyway based on the incoming electrical signal.
Accordingly this is a thing that you need to consider in your network --> Either to make both sides Auto, or configure both sides explicitly to avoid any Duplex mismatch problems.
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