Summary: This paper discusses the fixed-hub single allocation problem (FHSAP). In this problem, a network consists of hub nodes and terminal nodes. Hubs are fixed and fully connected; each terminal node is assigned to a single hub which routes all its traffic. The goal is to minimize the cost of routing the traffic in the network. In this paper, we propose a new linear programming (LP) relaxation for this problem by incorporating a set of validity constraints into the classical formulations by A. T. Ernst and M. Krishnamoorthy [Locat. Sci. 4, No. 3, 139–154 (1996; Zbl 0927.90065); Ann. Oper. Res. 86, 141–159 (1999; Zbl 0918.90096)]. A geometric rounding algorithm is then used to obtain an integral solution from the fractional solution. We show that by incorporating the validity constraints, the strengthened LP often provides much tighter upper bounds than the previous methods with a little more computational effort and the solution obtained often has a much smaller gap with the optimal solution. We also formulate a robust version of the FHSAP and show that it can guard against data uncertainty with little costs.