Why does ethtool show 'no link' for ethernet interface even though cable is physically connected?

Environment

• Red Hat Enterprise Linux
• Ethernet network interface

Issue

• The ethtool command shows Link detected: no for interface
• The ip link command shows UP but NO CARRIER
• The mii-tool command shows SIOCGMIIPHY on 'ethX' failed
• The network link light is on but the interface will not come up
• The message ADDRCONF(NETDEV_UP): ethX: link is not ready appears in kernel messages

Resolution

It's most likely there is a physical fault in the network interface, in the cable, or in the switchport. We suggest the following troubleshooting steps:

• Confirm that the switchport has been activated on the network switch.
• Try reseating the network cable at both ends to eliminate a bad connection.
• Try a new network cable to eliminate a faulty cable.
• Try another switchport to eliminate a faulty switchport.
• If you have a multi-port NIC, try another port on the NIC to eliminate that particular port on the multi-port card.
• Try another NIC to eliminate that particular card.
• Update NIC device to latest firmware.
• If after verifying the hardware connection, the hardware vendor may need to be engaged to verify there are no hardware related issues on the physical ports themselves.

Depending on the kernel version and network interface, there might be relevant fixes in the latest driver. You can update the driver by running yum update kernel to get the latest kernel, which contains the driver.

If you wish to view the kernel changelog to search for relevant fixes to your driver or network interface, you can install the yum-plugin-changelog package (yum-changelog on RHEL5) and run yum changelog <date> kernel, where <date> is the build date of your current kernel. You can find the build date of the current kernel with uname -v. You may also view the kernel changelog on RHN.

We also suggest updating the system BIOS and the NIC firmware.

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