What is an implicit-null label

The implicit-null label is label 3, which is used and assigned by an LSR. The implicit-null label, however, is not used in encapsulation. To switch the label, an LSR needs to pop up the label stack upon finding that the new label is an implicit-null label.

If the downstream forwarding entry uses label 3, the penultimate hop directly sends packets to the egress node without attaching any label. The egress node follows the IP forwarding process to forward the packets as if the packets contain no MPLS label.

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