Full/Half-duplex conflict detected

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

This behavior can, under some circumstances, appear very similar to rate-limiting behavior. Rate-limiting behavior is sensitive to the number of packets being sent. Increasing the number of packets per burst will more often trigger the rate limiting and help to confirm its presence.

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

Possible secondary messages

  • "May be rate-limiting behavior" Although a full/half-duplex conflict is more likely here, this condition should also be considered as a possibility.

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