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Environment/Versions Spirent TestCenter MPLS EXP bits Bound Streams Answer When creating the stream, before you click the Add button at the "select source and destination" window you need to check Show All Headers. Basically the MPLS EXP bits is set by setting the IP Header TOS/Diffserv values. See attached document. The higher order 3 bits of the IP TOS are copied to the EXP bits, irrespective of the manual changes in EXP  This is a standard behavior by default, explained in the RFC3270 section 3.5 (Also in https://www.cisco.com/en/US/tech/tk436/tk428/technologies_tech_note09186a008022ad7e.shtml) This mapping of the IP Precedence bits to/from the MPLS EXP bits upon ingress/egress is desired to preserve QOS between the IP/MPLS network boundaries. However with the use of modifiers in the stream configuration, we can achieve EXP value different from TOS value. Add a modifier for DSCP and EXP fields. Attachments Attachment Setting_MPLS_EXP.pptx