A duplex mismatch occurs when connected Ethernet interfaces use different duplex settings: one operates half duplex and the other full duplex. The link can remain up/up, but performance drops sharply. section 8.3.2 section 8.4
The mismatch can result from manually configuring opposite settings or enabling autonegotiation on only one side. For example, an autonegotiating interface that detects 100 Mbps selects half duplex, while a manually configured peer may remain at 100 Mbps full duplex. section 8.2 section 8.3.2
Recognize the operational symptoms:
- The half-duplex interface waits for the full-duplex device to stop transmitting before sending. section 8.3.2
- The half-duplex device may report false collisions and late collisions when both interfaces transmit. section 8.3.2
- Corrupted frames can increase Runts and CRC counters on the full-duplex device. section 8.3.2
- Use show interfaces to inspect collisions, late collisions, Runts, CRC errors, and other interface counters. section 8.3
Router-to-switch mismatch
R1 G0/1 uses autonegotiation while SW1 F0/1 is manually set to 100 Mbps/full duplex. R1 matches the speed but selects half duplex, leaving both interfaces operational while severely reducing link performance. section 8.2
