I think this totally depends on the MTU setting on the interface. MRU is usually advertised same as the MTU setting. If MTu is 1500 on the interface, it advertises MRU as nothing (as its default MTU). If MTU is set to 1492, MRU is advertised as 1492. You can see this by doing a debug ppp negotiation.
HTH
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus