Q. How do I find out my NIC speed in Linux ?
A. Use ethtool or mii-tool to display or change ethernet card settings.

ethtool

To display duplex speed of eth0, type the following command

# ethtool eth0

Sample outputs

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
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: on
        Supports Wake-on: pumbg
        Wake-on: g
        Current message level: 0x00000001 (1)
        Link detected: yes

mii-tool

To display duplex speed of eth0, type the following command

# mii-tool -v eth0

Sample outputs

SIOCGMIIREG on eth0 failed: Input/output error
eth0: negotiated 100baseTx-FD, link ok
  product info: vendor 00:50:43, model 11 rev 1
  basic mode:   autonegotiation enabled
  basic status: autonegotiation complete, link ok
  capabilities: 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD
  advertising:  100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD flow-control
  link partner: 100baseT4 100baseTx-FD 100baseTx-HD 10baseT-FD 10baseT-HD

Where,
100baseTx-FD: 100Mbps full duplex (FD)
100baseTx-HD: 100Mbps half duplex (HD)
10baseT-FD: 10Mbps full duplex (FD)
10baseT-HD: 10Mbps half duplex (HD)

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