A note on hierarchical hubbing for a generalization of the VPN problem

Robust network design refers to a class of optimization problems that occur when designing networks to efficiently handle variable demands. In this context, Fréchette et al. (2013) recently explored hierarchical hubbing: a routing strategy involving a multiplicity of "hubs" connected to terminals and each other in a treelike fashion. For a natural generalization of the VPN problem, we prove a structural characterization implying that the optimal hierarchical hubbing solution can be found efficiently, and relate this to a "Generalized VPN Conjecture".