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As a previous member indicated, if the particular STM32 device as an embedded Media Access Contorl (MAC) device, full implemenation of the Ethernet interface still will require an Physical Layer (PHY) device transceiver which acts as a bridge between the Link Layer (MAC) and the Physical Layer Media using one of several protocols including Media Independent Interface (MII), Reduced Media Independent Interface (RMII) or possibly Serial Media Independent Interface (SMII). A commonly used option is to utilize an external module with both the PHY device, such as a TI DP83848, Microchip LAN8720, STM ST802RT1A/B or Micrel KSZ8051MLL, the Magnetics (RJ45 Jack) and its own clock source typically (25MHz or 50MHz) depending on the protocol used.
Many current microcontrollers support RMII, as it requires about half the number of I/O pins compared to MII, which doubles its clocking requirements to 50MHz compared to MII's clocking requirement of 25MHz.