In an MPLS VPN environment, the QoS making over the MPLS backbone must be different from the one that is received from the customer. The forwarding on the egress PE is based on the marking that is…

MPLS implicit null (PHP) causes the MPLS label, and thus its EXP value, to be popped at the penultimate hop, preventing the egress PE from directly using the ingress PE's MPLS EXP marking for forwarding.

Implicit null (PHP) removes the MPLS EXP before the egress PE, and uniform mode propagates the customer's QoS marking, which contradicts the requirement for different backbone marking.

Uniform mode QoS propagates the customer's QoS marking into the MPLS backbone, which goes against the requirement for the backbone marking to be different.

While explicit null preserves the EXP to the egress PE, short pipe mode might still copy the IP DSCP to MPLS EXP at push, or it might not provide the full independent QoS handling required for the egress PE to forward solely based on the ingress PE's marking.

MPLS implicit null (PHP) causes the MPLS label, and thus its EXP value, to be popped at the penultimate hop, preventing the egress PE from directly using the ingress PE's MPLS EXP marking for forwarding.