Quick Answer

The correct order for penultimate hop popping steps begins with the egress LSR assigning the implicit-null label (value 3) to a specific FEC and advertising it upstream to its neighbor. This is because PHP optimizes MPLS forwarding by having the penultimate LSR pop the label before sending the packet, so the egress LSR receives a pure IP packet and performs an IP lookup directly, avoiding a double lookup. On the ENCOR 350-401 exam, this drag-and-drop sequence tests your understanding of MPLS label operations and the efficiency gain of removing the label at the penultimate hop. A common trap is confusing the egress LSR’s role—it does not pop the label but instead performs the final IP lookup. To remember the flow, think of the mnemonic “Assign, Advertise, Pop, Forward, Lookup,” which maps to the egress assigning the label, advertising it upstream, the penultimate router popping it, forwarding the unlabeled packet, and the egress doing the IP lookup.