A server is connected to a switch, and the server is experiencing erratic connectivity issues. Which of the following should be checked FIRST?

Routing is a Layer 3 concern and would typically cause complete loss of connectivity to remote networks rather than erratic local link behavior between a server and its directly connected switch.

Speed mismatches between a server and switch usually result in a complete link failure rather than erratic connectivity, because Ethernet cannot establish a link at all when speeds are incompatible.

A duplex mismatch causes the half-duplex side to use CSMA/CD collision detection while the full-duplex side transmits freely, resulting in late collisions, excessive retransmissions, and erratic throughput degradation. This is the first and most likely culprit for intermittent or erratic connectivity when a server is directly connected to a switch, as auto-negotiation failures can silently produce this mismatch.

NAT (Network Address Translation) is configured on a router or firewall and affects address translation for Internet-bound traffic, not the local physical link behavior between a server and a switch.