An implicit null label is the label with the value being 3. An LSR can use and distribute the implicit null label, whereas the label does not appear in an encapsulated packet. When an LSR is to swap labels and detects that the incoming label is an implicit null label, the LSR pops out the label from the label stack instead of swapping the label with a new label.
When detecting that the label with the value being 3 is to be assigned to the downstream entry, the penultimate hop directly forwards the packet to the egress without pushing any label into the packet. When receiving the packet and finding that the packet carries no label, the egress handles the packet as an ordinary IP packet.

useful for me

good doc

good