The exhibit shows that the egress router, PE-2, is advertising label 3 (Implicit Null) to the penultimate hop router (P). This instructs the P router to perform Penultimate Hop Popping (PHP), removing the outer MPLS label before forwarding the packet to PE-2. When the outer label is removed, the associated MPLS EXP bits, which carry the Class of Service (CoS) information, are also lost. Consequently, PE-2 receives a plain IP packet (with the inner VPN label) and cannot classify it based on the outer label's EXP bits. To resolve this, the explicit-null option must be configured on the egress router, PE-2. This causes PE-2 to advertise the Explicit Null label (label 0), preventing PHP and ensuring the MPLS header with the EXP bits is preserved until it reaches PE-2 for proper CoS classification.

A. Configure the explicit-null statement on PE-1.

Configuring explicit-null on the ingress router (PE-1) affects LSPs terminating on PE-1, not those originating from it. This will not solve the PHP issue at the egress (PE-2).

C. Configure VPN prefix mapping for the PE-1toPE-2 LSP.

VPN prefix mapping is used to direct specific VPN prefixes into a particular traffic-engineered LSP. It does not influence the label advertisement behavior (PHP vs. explicit-null).

D. Set a static CoS value for the PE-1toPE-2 LSP.

This would override any differentiated CoS markings with a single static value for all traffic in the LSP, which is contrary to the goal of preserving and acting upon the original CoS markings.

1. Juniper Networks TechLibrary

MPLS Applications User Guide

"Configuring Explicit Null Labels for LSPs": "By default

penultimate-hop popping is enabled. When PHP is enabled

the quality-of-service (QoS) information is lost because the MPLS header is removed by the penultimate-hop router. The egress router is unable to examine the EXP bits in the MPLS header. To preserve the QoS information for a packet

you can disable PHP. The MPLS header and the QoS information are then available on the egress router for processing." This is achieved by configuring explicit-null on the egress router.

2. Juniper Networks TechLibrary

Junos OS CoS for Routers

"CoS on MPLS LSPs Overview": "For the egress PE router to classify a packet based on the EXP value

the packet must arrive at the egress PE router with the MPLS label stack intact. By default

the penultimate router pops the outer label. To prevent the penultimate router from popping the outer label

you must configure the explicit null label on the egress PE router."

3. Juniper Networks Day One: Deploying MPLS

Chapter 5: Class of Service

Page 65: "The problem with PHP is that the EXP bits are lost when the label is popped... To solve this problem

you can configure the egress PE router to advertise an explicit NULL label (label value 0) instead of an implicit NULL label (label value 3). This forces the penultimate router to send the labeled packet to the egress PE router

thus preserving the EXP bits."