During my time supporting a customer with big LAN contains hundreds of switches I’ve been told to hard set speed/duplex for each interface to reduce errors. Apparently this is not necessarily true. Hard-setting the speed/duplex won’t make the error go away.

Most internal LAN (access layer) has 10/100 and most of the problem often lies on bad cabling structure and not the interface itself.

Surprisingly, on 1GB interface auto-negotiation is a must due to

allow negotiation of master / slave PHY relationship for clocking at the physical layer. Without negotiation the line clock will not establish correctly and physical layers problems can result.

Greg Ferro posted a nice write-up about Autonegotiation on Ethernet – It Works, It Should Be Mandatory!