Symptoms: Duplex Mismatch

• Server legacy-app-01 (10.80.1.55) connected to switch sw-access-03 port Gi0/12 shows very poor network throughput.
• File transfers that should complete in seconds take minutes. A 100MB file transfer averages ~2 Mbps on a 1Gbps link.
• The interface error counters on the server show a high number of "collisions" and "late collisions."
• The switch port shows increasing "FCS errors" and "runts" counters.
• Ping to the server works but occasionally shows high jitter (1ms to 200ms variation).
• The problem has existed since the server was re-cabled after a data center move last week.
• The server team says they set the NIC to 1000Mbps full-duplex because "auto-negotiation is unreliable."
• The switch port is set to auto-negotiate.
• Link light on both sides shows solid green (link is up).