This utility checks or sets the status of a network interface’s Media Independent Interface (MII) unit. Most fast ethernet adapters use an MII to autonegotiate link speed and duplex setting.
Most intelligent network devices use an autonegotiation protocol to communicate what media technologies they support, and then select the fastest mutually supported media technology. The -A or –advertise options can be used to tell the MII to only advertise a subset of its capabilities. Some passive devices, such as single-speed hubs, are unable to autonegotiate. To handle such devices, the MII protocol also allows for establishing a link by simply detecting either a 10baseT or 100baseT link beat. The -F or –force options can be used to force the MII to operate in one mode, instead of autonegotiating. The -A and -F options are mutually exclusive.
The default short output reports the negotiated link speed and link status for each interface. If an interface or interfaces are not specified on the command line, then mii-tool will check any available interfaces from eth0 through eth7.
You can also manually set speed parameters with mii-tools:
mii-tool -F 100baseTx-FD eth1