What is a label-edged router (LER)?

A Label Edge Router (LER) is a router that operates at the boundary of an MPLS network. Its primary functions are to classify incoming IP packets into Forwarding Equivalence Classes (FECs), push a label onto them as they enter the MPLS domain (ingress LER), and pop the label as they exit the domain (egress LER). This role is precisely fulfilled by a Provider Edge (PE) router, which serves as the gateway between a customer's network (connected via a CE router) and the provider's MPLS core. The PE router handles the critical label imposition and disposition operations that define the edge of the label-switched path.

A. A CE (Customer Edge) router is located at the customer premises and connects to the PE router; it is typically unaware of the MPLS network and does not perform label operations.

B. A P (Provider) router is a core router, also known as a Label Switching Router (LSR), that performs high-speed label swapping in the MPLS backbone, but does not operate at the edge.

D. A DR (Designated Router) is a role within link-state routing protocols like OSPF, used to manage adjacencies on multi-access networks, and is unrelated to MPLS edge functions.

1. Rosen

E.

Viswanathan

A.

& Callon

R. (2001). Multiprotocol Label Switching Architecture. RFC 3031. Internet Engineering Task Force (IETF). In Section 2.1

"Label Edge Routers

" the document defines LERs as routers at the edge of an MPLS network responsible for pushing and popping labels. This function is architecturally identical to that of a PE router in an MPLS VPN context.

2. Davie

B.

& Venaas

S. (2008). MPLS Technology and Applications. Morgan Kaufmann. Chapter 2

"MPLS Architecture

" explicitly states: "The routers at the boundary of the MPLS cloud are known as Label Edge Routers (LERs)... In the context of MPLS/VPN services

these routers are also called Provider Edge (PE) routers."

3. Gredler

H.

& Medcalf

S. (2008). A VPLS-based Ethernet VPN Service for Carrier-Grade Networks. IEEE Communications Magazine

46(5)

138-145. https://doi.org/10.1109/MCOM.2008.4511657. The article describes the MPLS VPN architecture

where "Provider edge (PE) routers are label edge routers (LERs) that sit at the edge of a provider's core network..." (Section: VPLS Architecture

para. 1).