In LwIP HTTP server demo, ethernetif.c, HAL_ETH_MspInit():
...
/* Output HSE clock (25MHz) on MCO pin (PA8) to clock the PHY */
HAL_RCC_MCOConfig(RCC_MCO1, RCC_MCO1SOURCE_PLLCLK, RCC_MCODIV_4);
...
According to the schematic, this pin is the USB SOF input, and the LAN chip is clocked with a dedicated 25MHz crystal. Removing the code does not break Ethernet comms, so I'm inclined to trust the schematic rather than the code.
