LDP Explicit Null

The default label advertised by an LDP router that is the ultimate-hop router (FEC originator) is label 3 (Implicit Null label). If label 3 is advertised, the penultimate-hop router removes the label and sends the packet to the egress router with the top LDP label removed. If ultimate-hop popping is enabled, label 0 (IPv4 Explicit Null label) is advertised by the ultimate-hop router (FEC originator), and the penultimate-hop router swaps the top LDP label to label 0 (IPv4 Explicit Null label) for transit packets, when sending packets to the ultimate-hop router. Ultimate-hop popping ensures that any packets traversing an MPLS network include a label with MPLS traffic class (TC) bits, ensuring consistent QoS marking up to the ultimate-hop router.