Should network hardware be set to "autonegotiate" speeds or fixed speeds?

1. I have yet to see a problem with auto-negotiation of network speeds that isn't caused by either (a) a mismatch of manual on one end of the link and auto on the other or (b) a failing component of the link (cable, port, etc).

2. This depends on the admin, but my experience has shown me that if you manually specify the link speeds and duplex settings, than you are bound to run into speed mismatches. Why? Because it is nearly impossible to document the various connections between switches and servers and then follow that documentation when making changes. Most failures I have seen are because of 1(a) and you only get in to that situation when you start manually setting speed/duplex settings.

As mention in the Cisco documentation:

If you disable autonegotiation, it hides link drops and other physical layer problems. Only disable autonegotiation to end-devices, such as older Gigabit NICs that do not support Gigabit autonegotiation. Do not disable autonegotiation between switches unless absolutely required, as physical layer problems can go undetected and result in spanning tree loops.

Unless you are prepared to setup a change management system for network changes that requires the verification of speed/duplex (and don't forget flow control) or are willing to deal with occasional mismatches that come from manually specifying these settings on all network devices, then stick with the default configuration of auto/auto.

In the future, consider monitoring the errors on the switch ports with MRTG so you can spot these issues before you have a problem.

Edit: I do see a lot of people referencing negotiation failures on old equipment. Yes this was an issue a long time ago when the standards were being created and not all devices followed them. Are your NICs and switches less than 10 years old? If so, then this won't be an issue.

1. Very common, I've had numerous problems over the years with various types of hardware.

2. In my opinion if the setup is static(i.e. a server rack) and you don't think there will be changes it is a good idea to setup the speeds and duplexs manually. As long as it is well documented so that future problems can be averted.

EDIT:

Just to clarify, I am not advocating using manual speeds on your entire network, I would say that 95% of the time auto/auto is the way to go. I'm just saying I've had problems with duplex/speed and there are small portions of my network (i.e. one of our server racks ) that have mostly manual settings. We operate a very tightly controlled LAN with unused ports being shutdown and MAC-Filters on most of the ports so keeping track of the speeds is not very difficult.