Renegotiate ethernet card link speed / duplex

Due to software or hardware (network switch or ethernet card) error it may be possible that you need to restart auto-negotiation on the ethernet device. Here's how to do it:

mii-tool -r eth0

or

ethtool -r eth0

Then check the kernel log to see what happens:

% dmesg
eth0: link down
eth0: link up, 100Mbps, full-duplex, lpa 0x45E1

Thanks for this tip goes to Vivek.