STM32f107 and DP8384

The best way is to check some known good board as an example. Have a look at the https://www.st.com/content/st_com/en/products/evaluation-tools/product-evaluation-tools/mcu-mpu-eval-tools/stm32-mcu-mpu-eval-tools/stm32-eval-boards/stm3240g-eval.html#resource board - I know it's not 'F107, but the basics are the same. There are gerber files available where you can look at the layout (it's a compromise allowing both MII and RMII. RMII does not work properly on this board because the clock line (RMII_REF_CLK/PA1) is too long, as it goes not only from the oscillator to PHY and mcu, but also to the extension connectors. Cutting the piece of track which goes to the extension connector made it working OK. That should give you an estimate of how long the traces should be - as short as possible; say 10cm is probably the maximum you should be aiming for. The differences between them then are not that important. Note, that proper return (ground) is equally important; 4-layer board with dedicated ground layer is recommended.

Note also the series termination resistors on the signals.

JW