A duplex mismatch (for example, one end of the link in full-duplex and the other in half-duplex) can result in performance issues, intermittent connectivity, and loss of communication. It can also create subtle problems in applications. For example, if a web server is communicating to a database server through an Ethernet switch with a duplex mismatch, small database queries might succeed while large ones fail due to a timeout.
Manually setting the speed and duplex to full-duplex on only one link partner generally results in a mismatch. This common issue results from disabling auto-negotiation on one link partner and having the other link partner default to a half-duplex configuration, creating a mismatch. This is the reason why speed and duplex cannot be hard-coded on only one link partner. If your intent is not to use auto-negotiation, you must manually set both link partners' speed and duplex settings to full-duplex.