No network connectivity and ethtool shows NIC speed and duplex are unknown

Issue

• No network connectivity and ethtool reports link speed and duplex as unknown:

# ethtool eth0
Settings for eth0:
    Supported ports: [ FIBRE ]
    Supported link modes:   1000baseT/Full 
                            10000baseT/Full
    Supported pause frame use: Symmetric Receive-only
    Supports auto-negotiation: No
    Advertised link modes:  1000baseT/Full 
    Advertised pause frame use: No
    Advertised auto-negotiation: No
    Speed: Unknown!
    Duplex: Unknown! (255)
    Port: FIBRE
    PHYAD: 1
    Transceiver: internal
    Auto-negotiation: off
    Supports Wake-on: g
    Wake-on: g
    Current message level: 0x00000000 (0)
    Link detected: no