An explicit null label is the IPv4 explicit null label with the value being 0. The explicit null label is valid only at the bottom of a label stack, which indicates that an LSR must pop out the label and forward the packet based on the IPv4 header.
When detecting that a label with the value being 0 is assigned to the downstream forwarded entry, the penultimate hop pushes the label with the value being 0 into the label stack of the packet and then forwards the packet to the egress. When detecting that the label in the received packet is 0, the egress does not search the ILM but directly pops out the label and forwards the packet according to the IP header (assume that there is only one label in the label stack).

good note

Good

useful note

useful document, thanks