Cisco – Why is the link between the switch and the router always negotiating half-duplex mode

I changed the NIC settings on the PCs from auto-negotiate to 100/full. It didn't seem to help. Still some of the ports have negotiated to half-duplex.

If you forced the duplex/speed on a computer then you MUST also force the settings on the switch. In my experience most switches will default to a half-duplex connection. By forcing a duplex setting and disabling auto-negotiation on a computer, you are most likely causing a duplex-mismatch.

Here is a link describing the behavior for Cisco switches. Even Cisco switchs will default to 100/half if the connected computer is forced to 100/full.

If you can't manage the speed/duplex on the switch, then you should set everything to auto. With everything set to auto, if the duplex is not correctly negotiated, you probably should be looking at updating the firmware, contacting the vendor, or replacing the switch.

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.