Connecting an STM32 at Ethernet
I decided using SPI1 from the STM32 processor.

So, i mapped the physical connections as

SCS  -&gt; PA4
SCLK -&gt; PA5
MISO -&gt; PA6
MOSI -&gt; PA7


The trick was adding this line 

 Ethernet.init(PA4);


before the method.

Ethernet.begin(mac)


By default, the pin used for Arduino is PIN10.

By using Ethernet.init(PA4); we override that pin, with PIN14 (PA4), 

suitable for STM32 and have a working solution.