Questions to Determine

• What speed and duplex settings are configured on the server NIC?
• What speed and duplex settings are configured on the switch port?
• Is one side using auto-negotiation while the other is forced?
• What do the interface error counters show on both sides (collisions, FCS errors, late collisions, runts)?
• When one side forces speed/duplex, what does the auto-negotiating side fall back to?
• Has the cable been tested for physical integrity (e.g., bad crimp, wrong category)?
• What does ethtool eth0 show for link partner advertised capabilities?
• Are there any spanning-tree issues or port state changes logged on the switch?
• What was the configuration before the data center move?