If an implicit null label is assigned to the penultimate hop, it indicates that the egress allocates a label with the value being 3 for the penultimate hop. If a non-null label is assigned to the penultimate hop, it indicates that the PHP feature is not used. In this case, the egress allocates a normal label with the value equal to or larger than 16 for the penultimate hop.
Typical problem: The type of the label to be allocated is unchanged or is changed to non-null. In this case, no explicit null label is assigned to the penultimate hop.

Solution: Change the label distribution mode to explicit-null in the MPLS view.

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