This paper focuses on QoS guarantees in an interdomain selfish network where each domain may sell QoS guarantees for its transit traffic. The main objective of the paper is to evaluate the benefit for some of these domains to develop together a privileged partnership in terms of economic alliance. This alliance permits the members to share their local knowledge of the network and to exchange some traffic network services. After defining the alliance model and the way each domain may use it to obtain better QoS guarantees, we analyse by simulation on realistic generated topologies the impact of such alliances on the QoS requests satisfaction.