The most likely issue is a duplex mismatch, which is a well-documented cause of collisions and poor network performance, as outlined in the CompTIA Network+ N10-009 troubleshooting objectives. A duplex mismatch occurs when one end of a network link is configured for full duplex while the other end is operating in half duplex. In half-duplex mode, devices must take turns transmitting and receiving data, which inherently allows for collisions. When paired with a full-duplex device—which expects simultaneous send and receive—collisions and retransmissions occur frequently.

The presence of collisions on a switch interface is a key diagnostic clue. Modern Ethernet switches operating correctly in full duplex should not experience collisions at all. When collisions are observed, it almost always points to a duplex configuration issue, often caused by manual speed/duplex settings on one device and auto-negotiation on the other.

A wrong speed setting may prevent link establishment entirely or cause errors, but it does not typically result in collisions. Jumbo frames can cause packet drops or fragmentation issues, not collisions. An incorrect VLAN configuration affects logical segmentation and connectivity but operates at Layer 2 and does not generate collisions.

CompTIA Network+ emphasizes duplex mismatch as a classic and frequently tested troubleshooting scenario. Correcting it—usually by enabling auto-negotiation on both ends—restores normal, collision-free communication.