In which situation would you disable penultimate-hop popping?

By default, penultimate-hop popping (PHP) removes the MPLS label at the router just before the egress LSR. The egress router then receives a standard IP packet and must rely on the IP header's DSCP bits for Class of Service (CoS) classification. Disabling PHP (by signaling an explicit null label) ensures the MPLS-labeled packet is sent to the egress router. This preserves the MPLS header's Traffic Class (TC/EXP) bits, allowing the egress router to apply CoS policies based on the MPLS header. This enforces a consistent CoS behavior across the entire Label Switched Path (LSP).

A. Disabling PHP alters label operations on the penultimate router; it does not bypass the router. IPv6 tunneling capability is an unrelated feature.

B. The penultimate router forwards packets based on the MPLS label, not the final IP destination. The egress router performs the destination route lookup.

D. A router's support for IPv6 tunneling is irrelevant to the decision to disable PHP, which is a control plane signaling and forwarding plane action within MPLS.

1. Juniper Networks TechLibrary, Junos OS 22.4., "Understanding Penultimate Hop Popping". This document states, "Disabling PHP is useful...if you need to preserve the MPLS label and EXP bits for QoS purposes on the egress router." This directly supports the reasoning that disabling PHP is done to enforce CoS behavior.

2. Juniper Networks TechLibrary, Junos OS 22.4., "MPLS Applications Configuration Guide", section "Configuring an Explicit Null Label for LSPs". The guide explains that configuring no-penultimate-hop-popping causes the router to advertise an explicit null label, which "instructs the penultimate router not to pop the label." This ensures the label, with its CoS information, reaches the egress router.

3. Juniper Networks TechLibrary, Junos OS 22.4., "Class of Service Configuration Guide", section "MPLS and Class of Service Overview". This guide details how MPLS EXP bits are used for CoS. It implicitly supports the need to preserve these bits until the egress router for consistent QoS by explaining their role in packet classification and scheduling within an LSP. Disabling PHP is the mechanism to achieve this preservation on the final hop.