A crucial requirement for the network service provider is to satisfy the Service Level Agreements (SLA) that it has made with its customers. Coexisting network tenants may have agreed different SLAs, and thus, the service provider must be able to provide QoS differentiation in order to meet his contractual commitments. Current one-size-fits-all routing models are not appropriate for all network tenants if their individual SLA requirements are to be efficiently met. We propose a SDN-based multi-cost routing approach which allocates network resources based on a portfolio of tenant SLA, which achieves the goal of accommodating multiple tenants, given their SLAs. This routing approach allocates routes based on both the hop count and the probability of link failure. Experimental evaluation demonstrates that the assignment of network paths to tenants is prioritised according to the SLA class of the tenant. Differentiation between tenants who have different SLAs is achieved. Finally, we demonstrate how the routing model operates and how it impacts upon the provision of different levels of service.