Classification & Marking - @layer 2 MPLS

Packets entering an MPLS network have an additional 4-byte header added, this header contains the parameters for MPLS.

Within the MPLS header there is a field called EXP, this experimental field is used for marking priority. 3-bits are available for specifying priority, just like CoS field in 802.q frames, or IP precedence.

On the edge of the MPLS network, when an IP packet arrives the three most significant bits of the ToS (layer 3 QoS marking) are copied to the EXP field of the MPLS header. The three bits are called IP precedence.

It is possible for the MPLS service provider to chose not to copy the IP precendence to the EXP field, and instead configure the EXP field themselves, this leaves the ToS field available for the customer to mark as appropriate.