We had a case on one of our ESXi hosts equipped with an Intel Corporation 82571EB Gigabit Ethernet Controller – although it was 1Gbit in speed, we were unable to achieve autonegotiation higher than 100 Mbit. When setting it manually to 1Gbit, the NIC disconnected itself from the network. Every other setting worked – 10 Mbit and 100Mbit both half and full duplex. We tried investigating with our Network Team, forcing 1Gbit on switch and that has also brought the NIC down.
I delved deeper into this issue and observed the VMkernel log via tail -f when I have forcibly disconnected the NIC and reconnected it again via esxcli. One line appeared that caught my attention:
vmnic6 NIC Link is Up 100 Mbps Full Duplex, Flow Control: None e1000e 0000:07:00.0: vmnic6: Link Speed was downgraded by SmartSpeed e1000e 0000:07:00.0: vmnic6: 10/100 speed: disabling TSO
I immediately caugt up on SmartSpeed and tried to find a way to disable it – that is until I have found out on many discussion threads later that SmartSpeed is an intelligent throtlling mechanism that is supposed to keep the connection running on various link speeds when an error somewhere on the link path is detected. The switches were working okay, the NIC didn’t detect any errors, so the next thing to be checked would be the cabling.
I arranged a cable check with the Data Center operators and what do you know – replacing cables for brand new ones eventually solved the issue! Sometimes the failing component causing you a headache for a good few hours can be a “mundane” piece of equipment such as patch cables.