In an MPLS L3VPN architecture, the Provider (P) routers form the core of the service provider's network and are responsible for high-speed packet forwarding based on MPLS labels. They perform label switching on the transport label (outer label) to forward traffic between Provider Edge (PE) routers. P routers do not participate in the customer's routing via MP-BGP and do not maintain customer-specific routing tables (VRFs). Consequently, they have no knowledge of the customer's IP prefixes, ensuring the core remains simple, scalable, and secure.

A. A key feature of MPLS L3VPNs is the use of Virtual Routing and Forwarding (VRF) instances and Route Distinguishers (RDs), which specifically allows different customers to use overlapping IP address spaces.

B. MP-BGP is used to exchange VPNv4/VPNv6 routes between PE routers only (and Route Reflectors, if used). P routers are not involved in BGP and only need an IGP and LDP for label switching.

C. The connection between a PE and a CE router is a standard IP link. Routing information is exchanged using a standard routing protocol (e.g., static, OSPF, eBGP), not LDP, which operates within the MPLS core.

1. Rosen

E.

& Rekhter

Y. (2006). BGP/MPLS IP Virtual Private Networks (VPNs). RFC 4364. Internet Engineering Task Force.

Section 1

Introduction

Paragraph 4: "The P routers do not need to have any VPN routing information... This helps to ensure that the backbone can scale." This directly supports the correct answer (D) and refutes option (B).

Section 2

VRF

Paragraph 1: "If two VPNs use the same address space... a PE router must be able to install routes from both VPNs in its forwarding table..." This refutes option (A).

2. Davie

B.

& Farrel

A. (2008). MPLS: Next Steps. Morgan Kaufmann.

Chapter 3

Section 3.2.1

"The Roles of P and PE Routers": "The P routers are not aware of the VPNs and do not carry any customer routes. They simply switch labeled packets based on the information in their LFIBs." This confirms that P routers are unaware of customer routes (D).

3. Washington University in St. Louis

Course CSE 571S. (Fall 2013). MPLS and VPNs Lecture Notes.

Slide 25

"MPLS VPN Operation": Explicitly states

"P routers are not aware of VPN routes" and "PE routers peer with other PE routers using internal BGP (IBGP)." This supports answer (D) and clarifies the role of MP-BGP

refuting (B).