Network interface speed is 100Mbps and should be 1Gbps

Issue

• Network interface speed is 100Mbps and should be 1Gbps.
• ethtool command shows less Speed than Advertised and Supported link modes.
• 1000Mbps NIC is not auto-negotiating to full gigabit speed but running at 10 or 100 megabit.
• Network card is not working at its proper speed.

# ethtool eth0

Settings for eth0:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Advertised auto-negotiation: Yes
        Speed: 100Mb/s
        Duplex: Full
        Port: Twisted Pair