What MPLS function should be performed by a router that receives a data packet containing an MPLS label value of 3?

MPLS label value 3 is the "Implicit NULL" label, a special-purpose label defined in RFC 3032. An egress Label Edge Router (LER) signals this label value to its upstream neighbor (the penultimate hop). This instructs the penultimate router to perform Penultimate Hop Popping (PHP), removing the MPLS label before forwarding the packet to the egress LER. Consequently, the packet arrives at the egress LER as a native IP packet without any MPLS label. Therefore, a router will never receive a data packet on the wire with the label value of 3 in its header.

B. This is not a standard MPLS function. Label requests are part of the control plane, not a data plane reaction to a specific label value.
C. The penultimate hop router pops the label, but the egress router never receives a packet with label 3 to pop.
D. Pushing a label is an ingress or transit function, not a reaction to receiving the Implicit NULL label, which signals the end of an LSP.
E. Signaling is a control plane function. A router does not signal a new label in response to receiving a specific data packet.

1. IETF RFC 3032, "MPLS Label Stack Encoding", Section 2.1, "Reserved Label Values": "Value 3 represents the Implicit NULL Label. This label value is the default... It indicates that the label stack must be popped, and the forwarding of the packet must be based on the IP header."

2. Nokia 7750 SR OS MPLS Guide, Release 21.10.R1, "MPLS Label Manager" chapter, "Implicit Null Label" section: "The implicit null label is a label with a value of 3. An LER advertises this label for a specific FEC to its upstream LDP peers. This causes the upstream LSR to perform a pop operation..."