Since the 878 router doesnt have any optical ports on it, you will have to termiate that link on a local switch (if the switch has optics on it).

If the switch also doesnt the compatible optics/or no optics then your best best to to purchase a good quality media convertor and connect the incoming fiber to the router via copper (this wont disable or degrade any functionality).

HTH-Cheers,

Swaroop