Flexible optical network (FON) architectures are considered a very promising solution where spectrum resources are allocated within flexible frequency grids. These architectures must handle the spectrum more efficiently and must support on-demand elastic bandwidth provisioning. However, when multi-bandwidth services coexist without a suitable bandwidth management, blocking of wider spectrum calls increases compared to narrower spectrum calls. This unfairness effect limits FONs' performance, not fulfilling much of the promise they hold. In this letter we present a new specific variant of trunk reservation policies that can effectively deal with fast on-demand bandwidth provisioning. Through network simulations we show that trunk reservation is able to guarantee fairness and to equalize the performance of an entire network in terms of blocking probability and link utilization with different flexible spectrum allocation policies.