In Multiprotocol Label Switching (MPLS), a Label Switch Router (LSR) performs one of three primary actions: push, swap, or pop. The exhibit shows that router R4 receives a packet with an incoming label of 40. It then replaces this label with an outgoing label of 50 before forwarding the packet to the next-hop router, R5. This process of replacing an incoming label with a new outgoing label is known as the "swap" operation. This is the standard function of an intermediate LSR within an MPLS domain that is not the penultimate hop performing Penultimate Hop Popping (PHP).

A. pop: The pop operation, which removes the label, is performed by the egress router R5 in this diagram, not by R4.

B. push: The push operation, which adds the initial label, is performed by the ingress router R1 at the start of the MPLS path.

C. PHP: Penultimate Hop Popping (PHP) is a specific action where the second-to-last router (R4) would pop the label. The exhibit explicitly shows R4 swapping labels, not popping one.

1. Rosen

E.

Viswanathan

A.

& Callon

R. (2001). RFC 3031: Multiprotocol Label Switching Architecture. IETF. Section 3.12

"The Label Swap Operation

" states: "A label swap operation consists of replacing the label at the top of the label stack with a new label

and then forwarding the resulting packet." This directly describes the action of R4.

2. Davie

B.

& Peterson

L. (2011). Computer Networks: A Systems Approach (5th ed.). Morgan Kaufmann. In Chapter 3.3.1

"Operations

" the text describes the function of an LSR: "An LSR

on receiving a labeled packet

uses the label as an index into a table that specifies the outgoing label and the next hop for the packet. It then swaps the label in the packet with the outgoing label..."

3. Gredler

H.

& Medcalf

S. (2008). MPLS and VPN Architectures. Cisco Press. Chapter 2

"MPLS Architecture

" describes the label operations

clarifying that intermediate LSRs perform label swapping to forward traffic along a Label Switched Path (LSP).