In an L3VPN using OSPF as the PE-CE protocol, the PE router advertises routes learned from the remote site (via MP-BGP) to the local CE router as OSPF external Type 5 LSAs. The PE router effectively acts as an OSPF ASBR. In the given scenario, CE-1 learns routes to Site-2 from two paths:

1. As external routes from PE-1.

2. As intra-area routes directly from CE-2 over the backdoor link.

According to the OSPF route selection algorithm, intra-area routes are always preferred over external routes, regardless of the metric. Therefore, by default, CE-1 will select the direct link to CE-2 as the best path to reach Site-2.

A. This describes a solution to make the L3VPN the primary path, not the default behavior. The backdoor link is preferred by default.

C. This is incorrect because OSPF's route preference rules prioritize the intra-area path (direct link) over the external path (L3VPN).

D. Increasing the metric on the PE-1 link would make the L3VPN path even less desirable, reinforcing the default preference for the direct link.

1. Juniper Networks TechLibrary

Junos OS VPNs Library for Routing Devices

"OSPF Sham Links Overview": This document explains the problem of OSPF backdoor links being preferred over the L3VPN backbone. It states

"By default

OSPF prefers an intra-area backdoor route over an inter-area route through the VPN backbone." This confirms that the direct link (an intra-area path) is preferred over the L3VPN path (which presents as an inter-area or external path).

2. Juniper Networks TechLibrary

Junos OS VPNs Library for Routing Devices

"Configuring OSPF in L3 VPNs": This section details the PE-CE OSPF interaction. It clarifies that "When you configure OSPF as a PE-to-CE routing protocol

the PE router advertises the routes it learns from the CE router to other PE routers through IBGP... The other PE routers forward these routes to their CE routers." These routes are injected into the CE's OSPF domain as external routes.

3. RFC 4577: OSPF as the Provider/Customer Edge Protocol for BGP/MPLS IP Virtual Private Networks (VPNs)

Section 4.2.5.1: This RFC

which defines the standard behavior

states: "If there is a "backdoor" link between two sites of a VPN

OSPF will prefer this link over the VPN backbone path if the backdoor path is an intra-area or inter-area path." This is because the path over the VPN backbone is treated as a less-preferred inter-area or external path.