$ Linux Command

Network

mii-tool

View and configure Media Independent Interface (NIC) settings.

networknicethernetlinkspeedduplex

Additional Notes

mii-tool checks and configures the Media Independent Interface (MII) on Ethernet network interface cards. It displays link status, speed, duplex mode, and autonegotiation settings.

It is useful for diagnosing physical layer issues: a down link may indicate a disconnected cable, a duplex mismatch can cause poor performance, and inconsistent negotiation between the NIC and the switch can lead to connectivity problems. On modern systems, ethtool is the preferred replacement.

Syntax

mii-tool [options] [interface...]

Parameters

  • interface: Network interface name (e.g., eth0). If omitted, all interfaces are shown.

Common Options

  • -v, --verbose: Show detailed MII information.
  • -V, --version: Show version information.
  • -r, --restart: Restart autonegotiation.
  • -F media, --force media: Force the interface to a specific media type (e.g., 100baseTx-FD, 10baseT-HD).
  • -w, --watch: Watch the interface for link status changes.
  • -p, --phy: Show PHY (physical layer) register information.

Examples

mii-tool

Show link status for all interfaces.

mii-tool eth0

Show status for eth0.

mii-tool -v eth0

Show detailed MII information.

mii-tool -F 100baseTx-FD eth0

Force 100 Mbps full-duplex mode.

mii-tool -r eth0

Restart autonegotiation on the interface.

mii-tool -w eth0

Monitor link status in real time.

Practical Notes

  • mii-tool works only with MII-compliant NICs. Many modern NICs do not support it.
  • ethtool is the modern replacement and supports many more NICs and features.
  • Forcing speed and duplex with -F disables autonegotiation. This can cause a mismatch if the switch does not match the forced settings.
  • A link showing no autonegotiation and link ok is typical when the device is forced to a specific setting.