It is any way to setup the Ethernet port speed and communications mode ?
I like to have the speed to be setup to 100Mbps Full.
Were I can do this ?
Gigabit Ethernet fix speed and full-duplex
I don't have any experience with what you're asking about. I was under the impression the speed of most Ethernet systems is negotiated actively since various components (like using multiple hubs) can introduce subtle electrical effects that create problems as you go full speed. Googling suggested there's a program called 'ethtool' one can explicitly use for the purpose of recommending a speed to the adapter.
Is this for a MangoES? If so, the following link may help: https://wiki.debian.org/NetworkConfiguration#Setting_the_speed_and_duplex
As noted in that link, do be careful (particularly if this device is remote!) that you do not inadvertently lock yourself out of the device. Unstable link speeds can often be symptomatic of bad cabling, faulty termination, or intermittent noise and identifying and addressing the root cause may be called for.
One other thing to note is that many switches and other bits of network gear allow you to set a 'manual' speed by effectively disabling some rates, eg. you could disable Gigabit on the switch port and auto-negotiation would max out at 100Base-TX.