Question

A service provider runs LDP across its core. The egress PE (loopback 10.0.0.5/32) advertises a label mapping to its penultimate-hop router. The QoS team complains that on MPLS-transported voice traffic, the EXP-based marking is being lost right before packets reach the egress PE, causing inconsistent per-hop behavior on the final PE. They want the penultimate router to keep pushing a labeled packet to the egress PE so the EXP bits survive to the last hop, while still avoiding a double IP lookup penalty on the egress. Which label-signaling behavior should the egress PE advertise to achieve this?

Accepted Answer

Advertise the explicit-null label (value 0) so the penultimate router still imposes a label carrying EXP bits to the egress PE

Answer

Advertise the implicit-null label (value 3) to trigger penultimate hop popping and remove the label one hop early

Answer

Advertise an aggregate label so the egress PE performs a full FIB lookup and re-marks DSCP from scratch

Answer

Disable LDP on the egress PE link and rely on IP forwarding to preserve the QoS marking