Oscillator and output load
There are at least two popular solutions:

two RMII clock slaves (both PHY and MAC)


one RMII clock master (PHY) and one RMII clock slave (MAC)



Other variants are possible. For example, if your MCU could be clocked by a 50 MHz source, you can use a variant with the MAC as RMII clock master and the PHY as RMII clock slave.