Ethernet data travels effectively from one MAC to another MAC with two Ethernet PHYs in between. Each of these 4 ICs can have their own reference clocks and crystal attached to each is the usual source of this reference clock. Most commonly used crystals are 25MHz crystals with a certain worst case frequency deviation captured as ppm shift. While designing this overall Ethernet system it is important to choose the crystals components such that constraints of both MAC and PHY are taken care of. IEEE and XMII standards also help in deriving the required ppm specification of these crystals.
This FAQ explains the following for RGMII/SGMII/RMII modes of operation for Ethernet sub-system and TI's automotive PHYs (DP83TG720, DP83TC811, DP83TC812, DP83TC814):
1. Pictorial representation of different clock domains
2. Where will the contention be because of ppm difference?
3. Which specification should be checked to resolve the contention?
4. Commonly used crystal ppm spec supported by most across the components.