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The ethernet controller is the bridge between the ethernet and the microcontroller that uses either PCIe or RGMII for further data transmission. They use an external oscillator as a reference clock to transmit data to PCIe/RGMII. It is also common to use an EEPROM for non-volatile data storage and to program the GPIO pins.
Resources:
https://ww1.microchip.com/downloads/en/DeviceDoc/LAN7430-LAN7431-Data-Sheet-DS00002631D.pdf