Somewhere in your building there is a PC whose adapter was pinned to an Mbps setting years ago by a forgotten template, a copied image or a well-meant tweak. Against a modern gigabit switch, that one setting quietly produces exactly the complaint nobody can reproduce: works, drops, works.
Auto-negotiation is not a suggestion; on gigabit Ethernet it is part of how the link organises itself, including the master and slave timing roles the two ends must agree on. The classic disaster is one-sided forcing: an adapter or switch port hard-set to a speed and duplex while its partner is left on Auto. The Auto side can sense the speed from the signal, but it cannot sense the duplex - so by the rules of the standard it must assume half duplex. Now one end talks whenever it wants while the other politely waits for silence. That is a duplex mismatch, and it drops packets precisely when traffic gets busy.
Example only: the forced side never learns the truth and the auto side is forbidden to guess full duplex. Both ends report a link that is up - and both are half right.
A duplex-mismatched link passes a login screen, opens a small file, and answers most pings. Collisions only pile up when both ends transmit together - bigger transfers, busy hours, several users. The half-duplex end backs off and retries; the full-duplex end sees runts and CRC errors. Users report an application that reconnects, stalls or corrupts transfers under load, while every idle-time test looks clean. If your fault respects working hours, put duplex on the suspect list early.
In the video case the adapter was sitting at an Mbps value against a gigabit switch - the same family of fault, cured in the settings, not the walls.
Rarely, and only in pairs: some ancient industrial gear and old media converters genuinely cannot negotiate, and the standing rule there is force both ends to the identical speed and duplex and document it on the port label. Forcing one end only is never a fix - it is how mismatches are manufactured. And note: gigabit itself is normally left to negotiate even in forced setups, because 1000 Mbps operation depends on negotiation for its timing roles.
One PC in the building with a forced setting usually means the image or the template carries it - audit before it breeds. On Windows, script the check across machines: pull each adapter advanced setting with PowerShell, or simply make the link speed report part of your helpdesk script. A fleet reading of link speeds will show you 100 Mbps islands on a gigabit network, and every island is either a genuinely old device or a setting waiting to become a ticket.
See the video case - the Mbps setting hiding in a gigabit office, found only after the cable and jack were cleared.
Watch Full VideoOne end of the link runs full duplex while the other runs half duplex. The half-duplex side treats simultaneous traffic as collisions and backs off; the full-duplex side receives damaged frames. The link stays up, light traffic passes, and packets drop as load rises.
When a port is forced, it stops sending negotiation signals. The Auto partner can detect the speed from the raw signal but has no way to learn the duplex, and the Ethernet standard requires it to fall back to half duplex in that situation. The mismatch is created by the rules, not by faulty hardware.
Only when a device truly cannot negotiate, and then force both ends to the identical setting and label it. For ordinary PCs and gigabit switches, leave everything on Auto Negotiation - gigabit depends on negotiation for correct operation.