In frame-relay MPLS, where is the label imposed?

A.
Between Layer 1 and Layer 2 headers.

B.
Between Layer 2 and Layer 3 headers.

C.
In the VPI/VCU fields.

D.
In the VPI field alone.

Explanation:

MPLS labels are assigned to packets based on groupings or forwarding equivalency classes (FECs) at the ingress eLSR. A FEC is a group of packets from a source IP address that are all going to the same destination. The MPLS label is imposed between Layer 2 and Layer 3 headers in a frame-based packet environment, or in the Layer 2 virtual path identifier/virtual channel identifier (VPI/VCI) field in cell-based networks like ATM.