Multiprotocol Label Switching (MPLS) VPN service from service providers is commonly used by enterprises for WAN connectivity. The service is often available over different types of access links and usually offers multiple classes of service. The MPLS service generally provides good QoS and therefore, satisfies VoIP requirements. However, this service often depends on the Service Layer Agreement (SLA) and the actual quality delivered by the service provider.

With MPLS service, unlike private WAN, the enterprise controls QoS explicitly only on the access link, that is, on the connection from each enterprise site to the MPLS network. In the MPLS network, QoS is controlled by the service provider. The enterprise affects the service given to its traffic by assigning the traffic to appropriate classes of service in the service provider’s network. This is done with DiffServ Code Point (DSCP) marking in the packet’s IP header. DSCP remarking by the enterprise edge routers might be required. DSCP remarking includes mapping the DSCPs of enterprise traffic to the DSCP values designated by the MPLS service provider for the different classes of service in their service offering.