Troubleshooting Duplex Mismatches Affecting Physical Link Speed

Overview

This document serves as a comprehensive guide to troubleshoot duplex mismatch issues that affect physical link speed between Ethernet peers. It provides step-by-step instructions to diagnose and resolve problems, aiming to minimize downtime and improve user experience.

When interconnecting two Ethernet peers, using auto-negotiation is recommended, which allows Ethernet peers to exchange link speed and duplex information.

This article covers troubleshooting procedures for:

• Duplex mismatch issues between Ethernet peers
• Physical link speed degradation caused by incorrect duplex configuration
• Auto-negotiation configuration on Meraki Aps

Troubleshoot a duplex mismatch

Description: A duplex mismatch occurs when two interconnected Ethernet peers are operating with incompatible duplex settings. This results in packet loss due to collisions and degraded physical link speed.

Explanation:
Ethernet peers use electrical signals to determine when a link is established. These electrical pulses form a 16-bit word that defines the speed and duplex settings used during auto-negotiation.
When the speed and duplex settings of one Ethernet peer must be manually configured, it is important to configure the other peer with matching settings.
For example:

• Peer 1 = 100 Full duplex, Peer 2 = 100 Full duplex

 or

• Peer 1 = Auto-negotiation, Peer 2 = Auto-negotiation

Failing to configure interconnected Ethernet peers with matching duplex settings can result in a duplex mismatch. In a duplex mismatch condition, both peers may use the same wire pair to transmit data. Collisions can occur when both peers transmit electrical signals simultaneously.
These collisions can lead to:

• Packet loss
• Frame corruption
• Reduced network performance
• Reduced link speed

The following peer configuration combinations are known to cause this issue:

• Scenario 1:
  • Peer 1 = 100 Full duplex
  • Peer 2 = Auto-negotiation
  • Result: Peer 2 will be able to detect the 100Mbps speed but defaults to half duplex, which will cause packet loss due to collisions.
• Scenario 2:
  • Peer 1 = 100 Full duplex
  • Peer 2 = 100 Half duplex
  • Result: Peer 2 uses half duplex so collisions will occur.

Note: The Gigabit Ethernet standard requires auto-negotiation for both sides of a link. Gigabit peers should not be hardcoded to a particular speed or duplex.

Additional resources

For more information about duplex mismatch see: 

http://en.wikipedia.org/wiki/Autonegotiation

http://en.wikipedia.org/wiki/Duplex_mismatch

For more information on how to set your Meraki APs to auto-negotiation see:

Duplex negotiation on Meraki APs