Fill in the blanks: The device at the beginning of an LSP is referred to as the_________, and the device at the end of an LSP is referred to as the ___________.

In MPLS terminology, a Label Switched Path (LSP) is a unidirectional path from a starting point to an endpoint. The router at the beginning of the LSP, which performs the initial label PUSH operation on an IP packet, is called the ingress Label Edge Router (iLER). The router at the end of the LSP, which performs the final POP operation to expose the original IP packet, is called the egress Label Edge Router (eLER). Routers between the ingress and egress are known as transit Label Switching Routers (LSRs).

A. PE (Provider Edge) and P (Provider) are roles in an MPLS VPN context, not the fundamental terms for the start and end of any LSP.
B. An LER is a router at the edge, and an LSR is any MPLS-capable router. Both the start and end points are LERs, making this answer imprecise.
C. This option is imprecise for the same reason as B and lists the roles in a non-sensical order for a path.
E. This option reverses the correct order; the ingress (iLER) is the beginning, and the egress (eLER) is the end.
F. This option uses VPN terminology and reverses the typical data flow where a PE would be the ingress/egress.

1. IETF RFC 3031, "Multiprotocol Label Switching Architecture", Section 2.1, "Definitions". "The first LSR on an LSP is called the 'ingress LSR'; the last LSR on an LSP is called the 'egress LSR'." The terms iLER and eLER are industry-standard derivatives for ingress/egress Label Edge Routers.

2. Nokia 4A0-103 Multiprotocol Label Switching Student Guide, Module 1: MPLS Fundamentals. This module defines the core MPLS components, specifying the roles of the ingress LER (iLER) and egress LER (eLER) as the start and end points of an LSP.