DuplexMismatch

Duplex mismatch is a networking condition that occurs when two connected devices negotiate incompatible duplex settings, typically one end operating in full duplex while the other is in half duplex or where auto-negotiation fails. It is most common on Ethernet links that rely on auto-negotiation between devices such as switches, NICs, or media converters.

In practice, duplex mismatch leads to degraded performance characterized by low throughput, high latency, frequent retransmissions, and an increase in collisions on half-duplex segments. Modern networks with switches and full-duplex links are less susceptible, but legacy devices or improperly configured interfaces can still create problems, particularly on longer links or congested networks.

Causes include disabling auto-negotiation on one side, manual speed/duplex settings that do not match on both ends, connecting devices that do not support auto-negotiation, or switching between devices with differing duplex capabilities (for example, a hub or legacy switch on one end).

Diagnosis involves inspecting interface status on both ends, validating speed and duplex configurations, and looking for consistent indications of a single link partner. Tools include on-box status commands (for example, show interface or ethtool), error counters, and performance tests. Symptoms include retransmits, collisions, poor ping/throughput, and floods of late collisions in half-duplex mode.

Resolution usually requires aligning the duplex and speed settings on both ends. If auto-negotiation is supported, re-enable it on both sides. If auto-negotiation is unreliable, manually set both ends to the same speed and duplex (for example, 100 Mbps full duplex or 1 Gbps full duplex). After alignment, monitor performance and verify that collision and error counters return to baseline. In modern networks, keeping devices updated and avoiding hubs or legacy equipment on active links minimizes the risk of duplex mismatches.