When a value greater than 0 is set, configures the device to use the specified speed.
If "auto-negotiate" is "yes" the specified speed will be the only one advertised during link negotiation: this works only for BASE-T 802.3 specifications and is useful for enforcing gigabit speeds, as in this case link negotiation is mandatory. If the value is unset (0, the default), the link configuration will be either skipped (if "auto-negotiate" is "no", the default) or will be auto-negotiated (if "auto-negotiate" is "yes") and the local device will advertise all the supported speeds. In Mbit/s, ie 100 == 100Mbit/s. Must be set together with the "duplex" property when non-zero. Before specifying a speed value be sure your device supports it.