A device is exhibiting behaviour, including packet loss, that corresponds to a configuration contention in the last link of the path. Specifically, test data is suggesting half duplex upstream and full duplex downstream.

Recommended action

  • Ensure that all devices connected to hubs are set to half duplex.
  • Avoid using auto negotiation on network devices and NICs.
  • Ensure that switch duplex settings match the setting on the connected switch.

Detailed explanation

A conflict exists between Ethernet half duplex and full duplex. Under lightly-load conditions, the link may appear to be working properly. As traffic load and the number of concurrent network connections increases, the likelihood of packet loss increases.

If the conflict exists on a common server or in the core of a network, the most common symptom is poor response time during peak traffic periods. For client workstations, most applications work reasonably well, provided that 2 concurrent connections are never used. Therefore power users who tend to multi-task tend to be impacted harder.

If you receive secondary message "Duplex conflict likely but not certain", be aware that Delivery monitoring was not able to conclusively determine that the cause of packet loss is due to a duplex issue. However, one should look for duplex conflicts before looking for other causes of packet loss.

A duplex conflict always involves packet loss, and therefore this condition is serious in nature and should be investigated.

Possible secondary messages

  • "Duplex conflict likely but not certain"

Related topics