The high-priority forwarding classes are Network-Control (nc), Expedited (ef), High-1 (h1), and High-2 (h2). High-priority forwarding classes are always serviced at congestion points over other forwarding classes; this behavior is determined by the router queue scheduling algorithm. See Virtual hierarchical scheduling for more information.
With a strict PHB at each network hop, service latency is mainly affected by the amount of high-priority traffic at each hop. These classes are intended to be used for network control traffic or for delay- or jitter-sensitive services.
If the service core network is oversubscribed, a mechanism to engineer a path through the core network and to reserve bandwidth must be used to apply strict control over the delay and bandwidth requirements of high-priority traffic. In the router, RSVP-TE can be used to create a path defined by an MPLS LSP through the core. Premium services are then mapped to the LSP, with care to not oversubscribe the reserved bandwidth.
If the core network has sufficient bandwidth, it is possible to effectively support the delay and jitter characteristics of high-priority traffic without using traffic engineered paths, as long as the core treats high-priority traffic with the correct PHB.