It may be allowable. The relevant guidance is at FAR subpart 17.1, Multi-year Contracting and the corresponding DFARS supplementation at DFARS subpart 217.1. More specifically, FAR 17.104(a) states: "Multi-year contracting is a special contracting method to acquire known requirements in quantities and total cost not over planned requirements for up to 5 years unless otherwise authorized by statute..." If Dark Fiber services are covered by statute, the PMO should be able to explain this. Alternatively, FAR 17.105-1(b) lists conditions under which the agency head may authorize multi-year contracts. Perhaps this authorization already exists. In other words, you should inquire for the specific authorization for this intended multi-year contract with the PMO.