QoS mapping between IP packets and MPLS

The 3-bit EXP field in the MPLS header can be used to define a Class of Service (CoS) value for packets that traverse an LSP. The CoS value specifies a priority for MPLS packets.

There are two ways that a CoS value can be applied to packets that traverse an MPLS network through an LSP:

• A CoS value is manually configured for the LSP.
• No CoS value is set for an LSP, and the Type of Service (ToS) field in the IP header is used. In this situation, the device copies the first three bits in the ToS field of the packet to the CoS (EXP) field in the MPLS header. The ToS value maps to one of the four priority queues on the device.