What is a duplex mismatch and how does it affect network performance?

A duplex mismatch is one end full-duplex and the other half-duplex; communication still works but performance is very poor, which makes it hard to spot.

Duplex Operation and Mismatch Issues:

* One end full-duplex, the other half-duplex: the link still works but performs badly. Autonegotiation prevents it. *

Duplex Modes:

• Full-duplex: Can send and receive simultaneously
• Half-duplex: Can only send OR receive at one time

Key requirement: Interconnecting Ethernet interfaces must operate in the same duplex mode for best communication performance and to avoid inefficiency and latency.

Ethernet Autonegotiation:

• Facilitates configuration and minimizes problems
• Maximizes link performance between two interconnecting Ethernet links
• Connected devices first announce their supported capabilities
• Then choose the highest performance mode supported by both ends

Duplex Mismatch: Occurs when one device operates in full-duplex and the other in half-duplex.

Consequences:

• Data communication will still occur
• Link performance will be very poor
• May be difficult to troubleshoot because communication still happens

Common causes:

• Misconfigured interface (one end manually set to a fixed duplex while the other autonegotiates)
• Failed autonegotiation (rare)

Go deeper:

• Wikipedia — Autonegotiation — how two interfaces choose a common duplex, and the dedicated "duplex mismatch" section on why the link degrades.