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In optimal QoS provisioning all traffic flows are distributed among LSPs (label switching path) related to service classes. The congestion of concurrent flows (traversing the network simultaneously) can appear. For LSP creation the IGP (interior getaway protocol) uses simple on-line routing algorithms (e.g. OSPFS, IS-IS) based on shortest path methodology. In highly loaded networks it becomes insufficient technique. On other hand, constraint based explicit routing (CR) ensures traffic engineering (TE) capabilities. It may find a longer but lightly loaded path, taking care of congestion possibility. In this approach LSP can be pre-computed much earlier, possibly during SLA (service level agreement) negotiation process. An effective algorithm for congestion control is developed. It could be a very good solution for congestion avoidance and for better load-balancing purpose where links are running close to capacity. Significant improvements are possible through heuristic approach based on restrictions for capacity state value. Comparisons of results are discussed.