Diagnosing a 10/100 Ethernet Duplex Mismatch Despite Link Up

A 10/100 Ethernet link can report the correct speed while its two ends disagree about duplex operation. Small exchanges may work, making the connection appear healthy until traffic increases. Compare the actual resolved states at both ends before investigating packet buffers or replacing the cable.

Distinguish negotiation from forced operation

The Microchip PHY Speed and Duplex Configuration application note explains the interaction between auto negotiation, parallel detection, and forced 10/100 operation. A partner can detect speed without learning that the opposite end was forced to full duplex, producing mismatched duplex states.

Keep this behavior tied to the interface modes covered by the document. Do not assume an identical configuration rule applies to every other Ethernet physical interface or speed.

Record both ends of the link

Read each port’s administrative setting and actual resolved speed and duplex. Record whether negotiation completed and what capabilities were advertised. A user interface showing a requested setting is not always showing the active state.

Include the MAC configuration on an embedded board. The external PHY and the MAC must agree on the operating mode even when the copper link itself is established correctly.

Use traffic that exposes the problem

Compare a small one-way exchange with simultaneous traffic in both directions. Observe error, collision, and dropped frame counters appropriate to the devices. A mismatch can produce traffic dependent symptoms that a single successful ping does not reveal.

Clear or snapshot counters before each controlled test so historical errors do not obscure the result. Keep frame size, offered load, and test duration consistent when comparing configurations.

Restore a deliberate configuration

Use compatible negotiation settings or an explicitly matched forced configuration where the application requires one. Change both partners as a coordinated operation. Forcing only the local port to full duplex because it sounds faster can create the very mismatch being investigated.

After the change, restart the link using the supported procedure and read the resolved states again. Verify that startup software or switch management does not later restore an incompatible setting.

Save the partner identities, advertised capabilities, active modes, traffic conditions, and counter differences. A successful diagnosis connects the mismatch to measured traffic behavior and shows improvement after the modes agree. Link up and a displayed 100 Mbps value establish only part of the interface state; they do not prove that both ends handle simultaneous transmission using the same duplex rules. Record the switch port configuration used for the comparison.